diff --git a/.idea/workspace.xml b/.idea/workspace.xml
index 51b1ec8..2c4eb83 100644
--- a/.idea/workspace.xml
+++ b/.idea/workspace.xml
@@ -1,9 +1,9 @@
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       <provider selected="true" editor-type-id="text-editor">
-        <state relative-caret-position="264">
-          <caret line="45" column="0" lean-forward="true" selection-start-line="45" selection-start-column="0" selection-end-line="45" selection-end-column="0" />
+        <state relative-caret-position="360">
+          <caret line="10" column="31" lean-forward="true" selection-start-line="10" selection-start-column="31" selection-end-line="10" selection-end-column="31" />
           <folding>
             <element signature="e#0#18#0" expanded="true" />
           </folding>
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       </provider>
     </entry>
-    <entry file="file://$PROJECT_DIR$/SurPyval/parametric/regression/weibull.py">
+    <entry file="file://$PROJECT_DIR$/SurPyval/model/fitmodel.py">
       <provider selected="true" editor-type-id="text-editor">
-        <state relative-caret-position="231">
-          <caret line="43" column="0" lean-forward="true" selection-start-line="43" selection-start-column="0" selection-end-line="43" selection-end-column="0" />
+        <state relative-caret-position="432">
+          <caret line="12" column="18" lean-forward="true" selection-start-line="12" selection-start-column="18" selection-end-line="12" selection-end-column="18" />
           <folding>
-            <element signature="e#0#18#0" expanded="true" />
+            <element signature="e#0#28#0" expanded="true" />
           </folding>
         </state>
       </provider>
     </entry>
     <entry file="file://$PROJECT_DIR$/SurPyval/model/model.py">
       <provider selected="true" editor-type-id="text-editor">
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-          <caret line="24" column="23" lean-forward="false" selection-start-line="24" selection-start-column="22" selection-end-line="24" selection-end-column="23" />
+        <state relative-caret-position="518">
+          <caret line="55" column="36" lean-forward="true" selection-start-line="55" selection-start-column="36" selection-end-line="55" selection-end-column="36" />
           <folding>
             <element signature="e#0#35#0" expanded="true" />
           </folding>
         </state>
       </provider>
     </entry>
-    <entry file="file://$PROJECT_DIR$/SurPyval/model/fitmodel.py">
+    <entry file="file://$PROJECT_DIR$/SurPyval/node/node.py">
+      <provider selected="true" editor-type-id="text-editor">
+        <state relative-caret-position="615">
+          <caret line="80" column="36" lean-forward="false" selection-start-line="80" selection-start-column="36" selection-end-line="80" selection-end-column="36" />
+          <folding />
+        </state>
+      </provider>
+    </entry>
+    <entry file="file://$PROJECT_DIR$/SurPyval/parametric/fitted/exponential.py">
       <provider selected="true" editor-type-id="text-editor">
-        <state relative-caret-position="295">
-          <caret line="27" column="27" lean-forward="false" selection-start-line="27" selection-start-column="27" selection-end-line="27" selection-end-column="27" />
+        <state relative-caret-position="1080">
+          <caret line="52" column="28" lean-forward="false" selection-start-line="52" selection-start-column="28" selection-end-line="52" selection-end-column="28" />
           <folding>
-            <element signature="e#0#28#0" expanded="true" />
+            <element signature="e#0#18#0" expanded="true" />
+          </folding>
+        </state>
+      </provider>
+    </entry>
+    <entry file="file://$PROJECT_DIR$/SurPyval/parametric/regression/exponential.py">
+      <provider selected="true" editor-type-id="text-editor">
+        <state relative-caret-position="576">
+          <caret line="40" column="19" lean-forward="true" selection-start-line="40" selection-start-column="19" selection-end-line="40" selection-end-column="19" />
+          <folding>
+            <element signature="e#0#18#0" expanded="true" />
+          </folding>
+        </state>
+      </provider>
+    </entry>
+    <entry file="file://$PROJECT_DIR$/SurPyval/parametric/regression/weibull.py">
+      <provider selected="true" editor-type-id="text-editor">
+        <state relative-caret-position="858">
+          <caret line="39" column="103" lean-forward="false" selection-start-line="39" selection-start-column="103" selection-end-line="39" selection-end-column="103" />
+          <folding>
+            <element signature="e#0#18#0" expanded="true" />
           </folding>
         </state>
       </provider>
     </entry>
+    <entry file="file://$PROJECT_DIR$/SurPyval/parametric/fitted/weibull.py">
+      <provider selected="true" editor-type-id="text-editor">
+        <state relative-caret-position="810">
+          <caret line="35" column="39" lean-forward="true" selection-start-line="35" selection-start-column="39" selection-end-line="35" selection-end-column="39" />
+          <folding />
+        </state>
+      </provider>
+    </entry>
+    <entry file="file://$PROJECT_DIR$/README.md">
+      <provider selected="true" editor-type-id="split-provider[text-editor;markdown-preview-editor]">
+        <state split_layout="SPLIT">
+          <first_editor relative-caret-position="321">
+            <caret line="23" column="0" lean-forward="true" selection-start-line="23" selection-start-column="0" selection-end-line="23" selection-end-column="0" />
+            <folding />
+          </first_editor>
+          <second_editor />
+        </state>
+      </provider>
+    </entry>
   </component>
 </project>
\ No newline at end of file
diff --git a/README.md b/README.md
index e8aa4b4..d7fbde0 100644
--- a/README.md
+++ b/README.md
@@ -2,25 +2,32 @@
 
 # SurPyval
 
-SurPyval is a Bayesian survival analysis library
+SurPyval is a Bayesian survival analysis library.
+
+## Why use
+
+SurPyval is aimed at people who are using survival analysis libraries like lifelines, but who want more flexibility and access to Bayesian approaches.
+
+Users who are new to Bayesianism can make use of sensible defaults and helper methods, while power users can take extremely detailed control of models.
 
 ## Philosophy:
-    
-    * Models should be transparent about their assumptions and workings
-    * Models should allow tweaks and modifications
 
-Implementing this philosophy has a number of positive effects on the library:
+SurPyval is built on the core philosophy that it should be as easy as possible for users to understand and tweak models.  Many statistical libraries are really easy to use until you want a slightly different assumption, which they cannot support.
 
-    * The log-likihood and plate diagrams of models are exposed
-    * Models are created through composition of simple units
-    * SurPyval objects thinly wrap and expose well-know libraries (esp. scipy)
-    * There are no hand-offs to non-python objects
-    * Models allow for substitution of any of their composite blocks
-    
-The trade-off to get these goods is performance.  Models provide in the library are designed to be tweakable, which limits performance optimizations.  This manifests itself in a number of ways:
+There are two main architectural decisions this has entailed:
+
+#### Graph centric
+
+All models in SurPyval are build around graphical models.  Every model can return a plate diagram of its likelihood function.  Digger deeper, models are actually made through composing together different variable "Nodes".  To change the model, we can simply swap out or add nodes to the model's graph
+
+#### Thin wrapper over common libraries
+
+Allowing modification and tweaking is much less valuable if doing so requires learning a complex new API.  To make the process as simple as possible, most SuPyval classes and objects are relatively thin wrappers over classes from libraries like scipy and emcee.  SurPyval objects are eager to expose these common libraries to the user
+
+#### Trade offs
 
-    * Straight up crunching speed
-    * Memory useage
-    * Models often don't exploit conjugacy where it exists
+In general, SuPyval is comfortable with paying for composability and modifiability with performance.  For a lot of tasks, SurPyval won't run as quickly as (say) a custom written Stan model.  However:
     
-For very large data sets or very complicated models, you might be better off using something like Stan.
\ No newline at end of file
+    * For data sets with 6 figure rows, the slow down isn't much of a problem
+    * Any performance loss is asymetrrical, some use cases will be blazing fast
+    * There are ways of mitigating this (see performance part of docs)
\ No newline at end of file
diff --git a/SurPyval/model/fitmodel.py b/SurPyval/model/fitmodel.py
index 1c848e7..c8e5db4 100644
--- a/SurPyval/model/fitmodel.py
+++ b/SurPyval/model/fitmodel.py
@@ -1,6 +1,7 @@
 from typing import Dict, Any
+import numpy as np
 
-from SurPyval.node import NodeTree
+from SurPyval.node import NodeTree, Node
 from SurPyval.samplers import EmceeSampler
 
 
@@ -29,16 +30,29 @@ def generate_replicates(self, n_replicates: int):
         posterior_samples = self.posterior.sample(n_replicates)[:n_replicates]
         return [self.node_tree.generate_replicate(posterior_sample) for posterior_sample in posterior_samples]
 
-    def predict(self, data_dict: Dict[str, Any]):
-        fitted_node_tree = NodeTree(self.node_tree.node_dict, data_dict)
+    def predict(self, node_dict: Dict[str, Node]):
+        fitted_node_tree = self.node_tree.update(node_dict)
         fitted_model = FitModel(fitted_node_tree, self.posterior)
         return fitted_model
 
-    def sample_survival(self):
-        def plot_survival_function(surv_node):
-            start_point = surv_node.distribution.ppf( 0.005, **parsed_n )[0]
-            end_point = surv_node.distribution.ppf( 0.995, **parsed_n )[0]
-            x_s = np.linspace( start_point, end_point, 1000 )
-            vals = [surv_node.distribution.sf( x, **parsed_n )[0] for x in x_s]
-            from matplotlib import pyplot as plt
-            plt.plot( x_s, vals )
\ No newline at end of file
+    def survival_function(self, flat_parameter_array):
+        param_dict = self.node_tree.unflatten_parameter_array(flat_parameter_array)
+
+        def surv_function(**kwargs):
+            return self.node_tree["y"].sf(**{**param_dict, **kwargs})
+
+        return surv_function
+
+    def sample_survival_function(self, n_samples):
+        posterior_samples = self.posterior.sample(n_samples)[:n_samples]
+        return [self.survival_function(x) for x in posterior_samples]
+
+    def plot_survival_function(self):
+        param_dict = self.node_tree.unflatten_parameter_array(self.posterior.maximum_likihood)
+        start_point = self.node_tree["y"].ppf(0.005, **param_dict)
+        end_point = self.node_tree["y"].ppf(0.995, **param_dict)
+        x_s = np.linspace(start_point, end_point, 1000)
+        surv_function = self.survival_function(self.posterior.maximum_likihood)
+        vals = surv_function(y=x_s)
+        from matplotlib import pyplot as plt
+        plt.plot(x_s, vals)
diff --git a/SurPyval/model/model.py b/SurPyval/model/model.py
index 6f27d09..1c24d3f 100644
--- a/SurPyval/model/model.py
+++ b/SurPyval/model/model.py
@@ -1,5 +1,4 @@
 from scipy.optimize import minimize
-import emcee as em
 import numpy as np
 
 from SurPyval.node.tree import NodeTree
@@ -40,16 +39,8 @@ def fit(self, n_walkers: int =4, burn: int =500):
             The function has the side effect of populating self.posterior
         """
 
-        def generate_starting_points():
-            max_lik_point = self.maximum_likihood()
-            return [max_lik_point + np.random.normal(0, 0.01, int(self.node_tree.length())) for x in range(n_walkers)]
-        
-        ndim = self.node_tree.length()
-        sampler = em.EnsembleSampler(n_walkers, ndim, self.node_tree.logpdf)
-        p0 = generate_starting_points()
-        pos, prob, state = sampler.run_mcmc(p0, burn)
-        
-        posterior = EmceeSampler(sampler, pos)
+        posterior = EmceeSampler(self.node_tree.logpdf, self.maximum_likihood(), n_walkers)
+        posterior.sample(burn)
         return FitModel(self.node_tree, posterior)
 
     def maximum_likihood(self):
@@ -64,7 +55,13 @@ def maximum_likihood(self):
         array_like
             flat array of parameters
         """
-        neg_lok_lik = lambda *args: -self.node_tree.logpdf(*args)
-        result = minimize(neg_lok_lik, np.array([1] * self.node_tree.length()))
+
+        def neg_lok_lik(*args):
+            lik = -self.node_tree.logpdf(*args)
+            if lik is None or np.any(np.isnan(lik)):
+                return -np.inf
+            return lik
+
+        result = minimize(neg_lok_lik, np.array([1] * self.node_tree.length()), method='Nelder-Mead')
         max_lik_point = result["x"]
-        return max_lik_point
\ No newline at end of file
+        return max_lik_point
diff --git a/SurPyval/node/__init__.py b/SurPyval/node/__init__.py
index 963ea0a..b1ad294 100644
--- a/SurPyval/node/__init__.py
+++ b/SurPyval/node/__init__.py
@@ -1,2 +1,6 @@
 from SurPyval.node.node import Node, gamma, exponential, gaussian
-from SurPyval.node.tree import NodeTree
\ No newline at end of file
+from SurPyval.node.tree import NodeTree
+from SurPyval.node.datanode import DataNode
+from SurPyval.node.datalikihoodnode import DataLikihoodNode
+from SurPyval.node.transformation import DeterministicNode
+from SurPyval.node.parameter import ParameterNode
\ No newline at end of file
diff --git a/SurPyval/node/datalikihoodnode.py b/SurPyval/node/datalikihoodnode.py
index 5f6ca4c..0ceb6a5 100644
--- a/SurPyval/node/datalikihoodnode.py
+++ b/SurPyval/node/datalikihoodnode.py
@@ -1,4 +1,5 @@
 import numpy as np
+from typing import Dict, Any
 
 from SurPyval.node.node import Node
 
@@ -13,8 +14,12 @@ class DataLikihoodNode(Node):
     DataLikihoodNode automatically does this routing
     """
 
+    def __init__(self, distribution, data: Any, parameter_dict: Dict[str, str], constants_dict: Dict[str, Any]=None):
+        self.data = data
+        Node.__init__(self, distribution, parameter_dict, constants_dict)
+
     def logpdf(self, **kwargs):
         processed_kwargs = self.parse_unflattened_parameters(**kwargs)
-        likihood_contribution = self.distribution.logpdf(kwargs["y"], **processed_kwargs)
-        survival_contribution = self.distribution.logsf(kwargs["y"], **processed_kwargs)
+        likihood_contribution = self.distribution.logpdf(self.data, **processed_kwargs)
+        survival_contribution = self.distribution.logsf(self.data, **processed_kwargs)
         return np.sum(likihood_contribution[kwargs["event"].astype(bool)]) + np.sum(survival_contribution[~kwargs["event"].astype(bool)])
\ No newline at end of file
diff --git a/SurPyval/node/datanode.py b/SurPyval/node/datanode.py
new file mode 100644
index 0000000..089656e
--- /dev/null
+++ b/SurPyval/node/datanode.py
@@ -0,0 +1,34 @@
+from SurPyval.node import Node
+
+
+class DataNode(Node):
+    """
+    A Node for fixed sources of data in the model
+
+    These nodes don't contribute to the likihood function at all
+    Their only purpose is to store data for use in the model
+
+    Parameters
+    ----------
+    name: str
+          the name the data source has in the model (e.g. event, x)
+    data: array-like
+          the data itself, usually pd.DataFrame or np array
+
+    """
+
+    def __init__(self, name, data):
+        self.name = name
+        self.data = data
+
+    def sample(self, size=1, **kwargs):
+        return None
+
+    def logpdf(self, **kwargs):
+        return 0.0
+
+    def pdf(self, **kwargs):
+        return 1.0
+
+    def __str__(self):
+        return self.name
diff --git a/SurPyval/node/node.py b/SurPyval/node/node.py
index cfe605d..764443d 100644
--- a/SurPyval/node/node.py
+++ b/SurPyval/node/node.py
@@ -48,7 +48,7 @@ def __init__(self, distribution: rv_continuous, parameter_dict: Dict[str, str],
     def parse_unflattened_parameters(self, **kwargs):
         filtered_kwargs = {x: kwargs[x] for x in kwargs if x in self.parameter_names}
         renamed_kwargs = {self.parameter_dict[x]: filtered_kwargs[x] for x in filtered_kwargs}
-        return {**renamed_kwargs, **self.constants_dict}
+        return {**self.constants_dict, **renamed_kwargs}
 
     def logpdf(self, **kwargs):
         return self.distribution.logpdf(**self.parse_unflattened_parameters(**kwargs))
@@ -56,8 +56,11 @@ def logpdf(self, **kwargs):
     def pdf(self, **kwargs):
         return self.distribution.logpdf(**self.parse_unflattened_parameters(**kwargs))
 
-    def ppf(self, **kwargs):
-        return self.distribution.ppf(**self.parse_unflattened_parameters(**kwargs))
+    def sf(self, **kwargs):
+        return self.distribution.sf(**self.parse_unflattened_parameters(**kwargs))
+
+    def ppf(self, q, **kwargs):
+        return self.distribution.ppf(q, **self.parse_unflattened_parameters(**kwargs))
 
     def sample(self, size=1, **kwargs):
         """
diff --git a/SurPyval/node/parameter.py b/SurPyval/node/parameter.py
index 33ca199..2ab9602 100644
--- a/SurPyval/node/parameter.py
+++ b/SurPyval/node/parameter.py
@@ -23,3 +23,4 @@ def sample(self, **kwargs):
 
     def __str__(self):
         return self.name
+
diff --git a/SurPyval/node/tree.py b/SurPyval/node/tree.py
index 02fb6c6..8af2707 100644
--- a/SurPyval/node/tree.py
+++ b/SurPyval/node/tree.py
@@ -5,6 +5,7 @@
 from SurPyval.node.node import Node
 from SurPyval.node.transformation import DeterministicNode
 from SurPyval.node.datalikihoodnode import DataLikihoodNode
+from SurPyval.node.datanode import DataNode
 
 
 class NodeTree:
@@ -19,16 +20,38 @@ class NodeTree:
                    lookup from name to data (e.g. (event->np.array([True, False, True])
     """
 
-    def __init__(self, node_dict: Dict[str, Node], data_dict: Dict[str, Any]):
+    def __init__(self, node_dict: Dict[str, Node]):
         self.parameters: List[ParameterNode] = [x for x in node_dict.values() if type(x) is ParameterNode]
         self.transformations: List[DeterministicNode] = [x for x in node_dict.values() if type(x) is DeterministicNode]
         self.likihood_nodes: List[DataLikihoodNode] = [x for x in node_dict.values() if type(x) is DataLikihoodNode]
 
-        self.data_dict = data_dict
+        self.data_dict = {x[0]: x[1].data for x in node_dict.items() if type(x[1]) is type(x[1]) is DataNode}
         self.node_dict = node_dict
         self.node_names = sorted(node_dict.keys())
         self.flat_split_point = self.flattened_parameter_split_points()
 
+    def __getitem__(self, item: str):
+        return self.node_dict[item]
+
+    def update(self, updated_node_dict: Dict[str, Node]) -> 'NodeTree':
+        """
+        Upsert nodes in the tree
+
+        Nodes that are already in the tree will be updated
+        Original node_tree isn't modifed in the process
+
+        Parameters
+        ----------
+        updated_node_dict: Dict[str, Node]
+                           nodes to update or insert to the tree
+
+        Returns
+        -------
+        NodeTree
+            Updated NodeTree with new nodes
+        """
+        return NodeTree({**self.node_dict, **updated_node_dict})
+
     def append_transformations(self, parameter_dict):
         """
         Enrich the parameter dictionary with transformed parameters
diff --git a/SurPyval/parametric/fitted/exponential.py b/SurPyval/parametric/fitted/exponential.py
index e3a7f2d..d253e77 100644
--- a/SurPyval/parametric/fitted/exponential.py
+++ b/SurPyval/parametric/fitted/exponential.py
@@ -1,52 +1,28 @@
+import numpy as np
+import scipy.stats
 
-import seaborn as sns
+from SurPyval.node import NodeTree, gamma, DataNode, DeterministicNode, ParameterNode, DataLikihoodNode
+from SurPyval.model.model import Model
 
-from SurPyval.distributions.gamma import Gamma
-from SurPyval.core.sampling import NumpySampler
 
-
-def create_prior_from_deaths_and_total_observed_time(deaths, total_observed_time):
+def create_prior_from_deaths_and_total_observed_time(parameter_name, deaths, total_observed_time):
     alpha = deaths
     llambda = total_observed_time
-    return Gamma(alpha, llambda)
+    return gamma({parameter_name: "x"}, {"a": alpha, "scale": llambda})
+
 
-def create_prior_from_deaths_and_average_lifetime(deaths, average_lifetime):
+def create_prior_from_deaths_and_average_lifetime(parameter_name, deaths, average_lifetime):
     alpha = deaths
     llambda = average_lifetime * deaths
-    return Gamma(alpha, llambda)
-
-class DistributionNode:
-    
-    def __init__(self, name, prior_distribution, posterior_distribution):
-        self.name = name
-        self.prior = prior_distribution
-        self.posterior = posterior_distribution
-
-    def sample_prior(self, n_samples = 10000):
-        return self.prior.sample(n_samples)
-
-    def sample_posterior(self, n_samples = 10000):
-        return self.posterior.sample(n_samples)
+    return gamma({parameter_name: "x"}, {"a": alpha, "scale": llambda})
 
-class PosteriorPredictiveDistribution(NumpySampler):
 
-    def __init__(self, gamma_distribution):
-        self.distr = gamma_distribution
-
-    def sample(self, n_samples):
-        samples = self.distr.sample(n_samples)
-        posterior_predictive_samples = np.array(map(lambda x: np.random.exponential(1.0 / x), samples))
-        return posterior_predictive_samples
-
-    def plot(self, n_samples = 10000):
-        sns.distplot(self.sample(n_samples))
-
-class FittedExponential:
+class FittedExponential(Model):
     """
         Fit an exponential distribution to the life lengths
 
         Nodes:
-            * llambda - the main parameter for the expoential distribution
+            * alpha - the scale of the exponential distribution
             * y - predictive distribution for lifetime
 
         Likihood: 
@@ -59,21 +35,16 @@ class FittedExponential:
                 lambda => Total observed time
     """
 
-
-    def __init__(self, prior_dict, y, event):
-        self.prior_dict = prior_dict
-        self.d = np.sum(event)
-        self.y_sum = np.sum(y)
-        llambda_posterior = Gamma(prior.alpha + self.d, prior.llambda + self.y_sum)
-
-        self.constants = {"alpha_0": prior_dict["llambda"].alpha, "llambda_0": prior_dict["llambda"].llambda}
-        self.nodes = {
-            "llambda": DistributionNode("llambda", prior_dict["llambda"], llambda_posterior),
-            "y": DistributionNode("y", PosteriorPredictiveDistribution(prior_dict["llambda"]), PosteriorPredictiveDistribution(llambda_posterior))
+    def __init__(self, y, event, alpha_prior):
+        node_dict = {
+            "alpha": alpha_prior,
+            "y": DataLikihoodNode(scipy.stats.expon, y, {"alpha": "scale", "y": "x"}),
+            "event": DataNode("event", event)
         }
 
-    def fit(self):
-        pass
+        node_tree = NodeTree(node_dict)
+
+        Model.__init__(self, node_tree)
 
     @staticmethod
     def show_plate():
diff --git a/SurPyval/parametric/fitted/weibull.py b/SurPyval/parametric/fitted/weibull.py
index 92aa72f..bda2138 100644
--- a/SurPyval/parametric/fitted/weibull.py
+++ b/SurPyval/parametric/fitted/weibull.py
@@ -1,30 +1,9 @@
 import numpy as np
+import scipy.stats
 
-from SurPyval.node.parameter import Parameter
-from SurPyval.node.tree import NodeTree
+from SurPyval.node import NodeTree, Node, DataNode, DeterministicNode, ParameterNode, DataLikihoodNode
 from SurPyval.model.model import Model
 
-def likihood_distr(y, x, alpha, llambda):
-    if alpha <= 0 or llambda <= 0:
-        return - np.inf
-    return self.data["d"] * np.log(alpha) + self.data["d"] * llambda + (alpha - 1) * np.sum(self.data["event"] * np.log(self.data["y"])) - np.sum(np.exp(llambda) * self.data["y"] ** alpha)
-
-def survival_distr(y, x, alpha, llambda):
-    if alpha <= 0 or llambda <= 0:
-        return - np.inf
-    return self.data["d"] * np.log(alpha) + self.data["d"] * llambda + (alpha - 1) * np.sum(self.data["event"] * np.log(self.data["y"])) - np.sum(np.exp(llambda) * self.data["y"] ** alpha)
-
-class LikihoodNode(Node):
-    
-    def __init__(self, y, event, x, parameter_dict = {"alpha": "alpha", "llambda": "llambda"}):
-        self.parameter_names = parameter_dict.values()
-        self.parameter_dict = parameter_dict
-        self.distribution = DataLikihood(likihood_distr, survival_distr, y, event, x)
-
-    def sample(self, x, n_samples):
-        samples = np.exp(np.dot(x.T, self.distribution.sample(n_samples).T))
-        prior_predictive_samples = np.array(map(lambda x: np.random.exponential(1.0 / x), samples))
-        return prior_predictive_samples
 
 class FittedWeibull(Model):
     """
@@ -45,16 +24,18 @@ class FittedWeibull(Model):
             This means that fitting the model requires some numerical approximation, currently MCMC
     """
 
-    def __init__(self, prior_dict, y, event):
+    def __init__(self, y, event, alpha_prior, llambda_prior):
 
-        self.parameters = [Parameter("alpha", 1), Parameter("llambda", 1)]
-
-        self.node_dict = {
-            "alpha": prior_dict["alpha"],
-            "llambda": prior_dict["llambda"],
-            "y": LikihoodNode(y, event, x)
+        node_dict = {
+            "alpha": alpha_prior,
+            "llambda": llambda_prior,
+            "y": DataLikihoodNode(scipy.stats.weibull_min, y, {"alpha": "shape", "llambda": "scale", "y": "x"}),
+            "event": DataNode("event", event)
         }
-        self.node_tree = NodeTree(self.node_dict, self.parameters)
+
+        node_tree = NodeTree(node_dict)
+
+        Model.__init__(self, node_tree)
 
     @staticmethod
     def show_plate():
diff --git a/SurPyval/parametric/regression/exponential.py b/SurPyval/parametric/regression/exponential.py
index 41ef5ba..ddee7f3 100644
--- a/SurPyval/parametric/regression/exponential.py
+++ b/SurPyval/parametric/regression/exponential.py
@@ -1,12 +1,8 @@
 import numpy as np
 import scipy.stats
 
-from SurPyval.node.tree import NodeTree
-from SurPyval.node.node import Node
-from SurPyval.node.transformation import Transformation
+from SurPyval.node import NodeTree, Node, DataNode, DeterministicNode, ParameterNode, DataLikihoodNode
 from SurPyval.model.model import Model
-from SurPyval.node.parameter import Parameter
-from SurPyval.node.datalikihoodnode import DataLikihoodNode
 
 
 class ExponentialRegression(Model):
@@ -29,7 +25,7 @@ class ExponentialRegression(Model):
 
         Examples
         --------
-        >>> from SurPyval.node.node import gaussian
+        >>> from SurPyval.node import gaussian
         >>> import pandas as pd
         >>> beta_prior = gaussian({"beta": "x"}, constant_dict={"loc": 0.0, "scale": 100.0})
         >>> data = pd.DataFrame({"y": [1.0, 2.0], "x_0": [1.0, 1.0], "event": [1, 0]})
@@ -38,29 +34,19 @@ class ExponentialRegression(Model):
 
     def __init__(self, y, event, x, beta_prior):
 
-        data_dict = {
-            "y": y,
-            "x": x,
-            "event": event
-        }
-
-        transformations = [
-            Transformation(
-                lambda data_dict, parameter_dict: np.exp( np.sum( data_dict["x"] * parameter_dict["beta"], axis=1)),
+        alpha = DeterministicNode(
+                lambda data_dict, parameter_dict: np.exp(np.sum(data_dict["x"] * parameter_dict["beta"], axis=1)),
                 "alpha")
-        ]
 
         node_dict = {
-            "beta": beta_prior,
-            "y": DataLikihoodNode(scipy.stats.expon, {"alpha": "scale"})
+            "beta": ParameterNode(beta_prior, "beta", 1),
+            "alpha": alpha,
+            "y": DataLikihoodNode(scipy.stats.expon, y, {"alpha": "scale", "y": "x"}),
+            "x": DataNode("x", x),
+            "event": DataNode("event", event),
         }
-        parameters = [
-            Parameter("beta", 1)
-        ]
-        node_tree = NodeTree(node_dict,
-                             data_dict,
-                             parameters,
-                             transformations)
+
+        node_tree = NodeTree(node_dict)
 
         Model.__init__(self, node_tree)
 
diff --git a/SurPyval/parametric/regression/weibull.py b/SurPyval/parametric/regression/weibull.py
index bfef59b..ca6c7c1 100644
--- a/SurPyval/parametric/regression/weibull.py
+++ b/SurPyval/parametric/regression/weibull.py
@@ -1,12 +1,8 @@
 import numpy as np
 import scipy.stats
 
-from SurPyval.node.tree import NodeTree
-from SurPyval.node.node import Node
-from SurPyval.node.transformation import DeterministicNode
+from SurPyval.node import NodeTree, Node, DataNode, DeterministicNode, ParameterNode, DataLikihoodNode
 from SurPyval.model.model import Model
-from SurPyval.node.parameter import ParameterNode
-from SurPyval.node.datalikihoodnode import DataLikihoodNode
 
 
 class WeibullRegression(Model):
@@ -32,11 +28,6 @@ class WeibullRegression(Model):
     """
 
     def __init__(self, y, event, x, alpha_prior, beta_prior):
-        data_dict = {
-            "y": y,
-            "x": x,
-            "event": event
-        }
 
         llambda = DeterministicNode(
                 lambda data_dict, parameter_dict: np.sum(data_dict["x"] * parameter_dict["beta"], axis=1),
@@ -46,11 +37,12 @@ def __init__(self, y, event, x, alpha_prior, beta_prior):
             "beta": ParameterNode(beta_prior, "beta", 1),
             "alpha": ParameterNode(alpha_prior, "alpha", 1),
             "llambda": llambda,
-            "y": DataLikihoodNode(scipy.stats.weibull_min, {"alpha": "c", "llambda": "scale"})
+            "y": DataLikihoodNode(scipy.stats.weibull_min, y, {"alpha": "c", "llambda": "scale", "y": x}),
+            "x": DataNode("x", x),
+            "event": DataNode("event", event),
         }
 
-        node_tree = NodeTree(node_dict,
-                             data_dict)
+        node_tree = NodeTree(node_dict)
 
         Model.__init__(self, node_tree)
 
diff --git a/SurPyval/samplers/emceesampler.py b/SurPyval/samplers/emceesampler.py
index 176b5c1..9519cf4 100644
--- a/SurPyval/samplers/emceesampler.py
+++ b/SurPyval/samplers/emceesampler.py
@@ -1,14 +1,23 @@
+import emcee as em
+import numpy as np
 
 from SurPyval.samplers.sampler import Sampler
 
+
 class EmceeSampler(Sampler):
-    
-    def __init__(self, sampler, pos):
-        self.sampler = sampler
-        self.pos = pos
+
+    def __init__(self, lik_function, maximum_likihood, n_walkers=4):
+        self.lik_function = lik_function
+        self.n_walkers = n_walkers
+        self.maximum_likihood = maximum_likihood
+        self.pos = self.generate_starting_points()
+        self.sampler = em.EnsembleSampler(n_walkers, len(self.maximum_likihood), self.lik_function)
 
     def sample(self, n_samples):
         self.sampler.reset()
-        self.sampler.run_mcmc(self.pos, n_samples)
+        self.pos, prob, state = self.sampler.run_mcmc(self.pos, n_samples)
         return self.sampler.flatchain
 
+    def generate_starting_points(self):
+        max_lik_point = self.maximum_likihood
+        return [max_lik_point + np.random.normal(0, 0.01, len(self.maximum_likihood)) for x in range(self.n_walkers)]
diff --git a/SurPyval/samplers/npsampler.py b/SurPyval/samplers/npsampler.py
deleted file mode 100644
index df0b979..0000000
--- a/SurPyval/samplers/npsampler.py
+++ /dev/null
@@ -1,12 +0,0 @@
-import numpy as np
-
-from SurPyval.samplers.sampler import Sampler
-
-class NumpySampler(Sampler):
-    
-    def __init__(self, np_random, **kwargs):
-        self.np_random = np_random
-        self.kwargs = kwargs
-        
-    def sample(self, n_samples):
-        return self.np_random(size = n_samples, **self.kwargs).reshape(n_samples, 1)