diff --git a/src/observation.rs b/src/observation.rs
index 5582fb4..2dc7e54 100644
--- a/src/observation.rs
+++ b/src/observation.rs
@@ -26,3 +26,96 @@ pub fn f_params(elems: &[(usize, f64)], t: f64, storage: &Storage) -> (f64, f64)
 
     (m, v)
 }
+
+pub(crate) struct Core {
+    pub m: usize,
+    pub items: Vec<usize>,
+    pub coeffs: Vec<f64>,
+    pub indices: Vec<usize>,
+    pub ns_cav: Vec<f64>,
+    pub xs_cav: Vec<f64>,
+    pub t: f64,
+    pub logpart: f64,
+    pub exp_ll: f64,
+}
+
+impl Core {
+    pub fn new(storage: &mut Storage, elems: &[(usize, f64)], t: f64) -> Self {
+        Core {
+            m: elems.len(),
+            items: elems.iter().map(|(id, _)| id).cloned().collect(),
+            coeffs: elems.iter().map(|(_, sign)| sign).cloned().collect(),
+            indices: elems
+                .iter()
+                .map(|(id, _)| storage.get_item(*id).fitter.add_sample(t))
+                .collect(),
+            ns_cav: (0..elems.len()).map(|_| 0.0).collect(),
+            xs_cav: (0..elems.len()).map(|_| 0.0).collect(),
+            t,
+            logpart: 0.0,
+            exp_ll: 0.0,
+        }
+    }
+}
+
+impl Observation for Core {
+    fn match_moments(&self, mean_cav: f64, cov_cav: f64) -> (f64, f64, f64) {
+        todo!()
+    }
+
+    fn ep_update(&mut self, lr: f64, storage: &mut Storage) -> f64 {
+        // Mean and variance of the cavity distribution in function space.
+        let mut f_mean_cav = 0.0;
+        let mut f_var_cav = 0.0;
+
+        for i in 0..self.m {
+            let item = storage.item(self.items[i]);
+            let idx = self.indices[i];
+            let coeff = self.coeffs[i];
+
+            // Compute the natural parameters of the cavity distribution.
+            let x_tot = 1.0 / item.fitter.vs(idx);
+            let n_tot = x_tot * item.fitter.ms(idx);
+            let x_cav = x_tot - item.fitter.xs(idx);
+            let n_cav = n_tot - item.fitter.ns(idx);
+
+            self.xs_cav[i] = x_cav;
+            self.ns_cav[i] = n_cav;
+
+            // Adjust the function-space cavity mean & variance.
+            f_mean_cav += coeff * n_cav / x_cav;
+            f_var_cav += coeff * coeff / x_cav;
+        }
+
+        // Moment matching.
+        let (logpart, dlogpart, d2logpart) = self.match_moments(f_mean_cav, f_var_cav);
+
+        for i in 0..self.m {
+            let item = storage.item_mut(self.items[i]);
+            let idx = self.indices[i];
+            let coeff = self.coeffs[i];
+
+            let x_cav = self.xs_cav[i];
+            let n_cav = self.ns_cav[i];
+
+            // Update the elements' parameters.
+            let denom = 1.0 + coeff * coeff * d2logpart / x_cav;
+            let x = -coeff * coeff * d2logpart / denom;
+            let n = coeff * (dlogpart - coeff * (n_cav / x_cav) * d2logpart) / denom;
+
+            *item.fitter.xs_mut(idx) = (1.0 - lr) * item.fitter.xs(idx) + lr * x;
+            *item.fitter.ns_mut(idx) = (1.0 - lr) * item.fitter.ns(idx) + lr * n;
+        }
+
+        let diff = (self.logpart - logpart).abs();
+
+        // Save log partition function value for the log-likelihood.
+        self.logpart = logpart;
+
+        diff
+    }
+
+    fn kl_update(&mut self, _lr: f64, _storage: &mut Storage) -> f64 {
+        todo!();
+    }
+}
diff --git a/src/observation/ordinal.rs b/src/observation/ordinal.rs
index c57e5e0..370829d 100644
--- a/src/observation/ordinal.rs
+++ b/src/observation/ordinal.rs
@@ -1,7 +1,7 @@
 use crate::storage::Storage;
 use crate::utils::logphi;
 
-use super::{f_params, Observation};
+use super::{f_params, Core, Observation};
 
 pub fn probit_win_observation(
     elems: &[(usize, f64)],
@@ -26,37 +26,14 @@ fn mm_probit_win(mean_cav: f64, cov_cav: f64) -> (f64, f64, f64) {
 }
 
 pub struct ProbitWinObservation {
-    m: usize,
-    items: Vec<usize>,
-    coeffs: Vec<f64>,
-    indices: Vec<usize>,
-    ns_cav: Vec<f64>,
-    xs_cav: Vec<f64>,
-    _t: f64,
-    logpart: f64,
-    _exp_ll: usize,
+    core: Core,
     margin: f64,
 }
 
 impl ProbitWinObservation {
     pub fn new(storage: &mut Storage, elems: &[(usize, f64)], t: f64, margin: f64) -> Self {
-        /*
-        assert len(elems) > 0, "need at least one item per observation"
-        */
-
         ProbitWinObservation {
-            m: elems.len(),
-            items: elems.iter().map(|(id, _)| id).cloned().collect(),
-            coeffs: elems.iter().map(|(_, sign)| sign).cloned().collect(),
-            indices: elems
-                .iter()
-                .map(|(id, _)| storage.get_item(*id).fitter.add_sample(t))
-                .collect(),
-            ns_cav: (0..elems.len()).map(|_| 0.0).collect(),
-            xs_cav: (0..elems.len()).map(|_| 0.0).collect(),
-            _t: t,
-            logpart: 0.0,
-            _exp_ll: 0,
+            core: Core::new(storage, elems, t),
             margin,
         }
     }
@@ -68,69 +45,25 @@ impl Observation for ProbitWinObservation {
     }
 
     fn ep_update(&mut self, lr: f64, storage: &mut Storage) -> f64 {
-        // Mean and variance of the cavity distribution in function space.
-        let mut f_mean_cav = 0.0;
-        let mut f_var_cav = 0.0;
-
-        for i in 0..self.m {
-            let item = storage.item(self.items[i]);
-            let idx = self.indices[i];
-            let coeff = self.coeffs[i];
-
-            // Compute the natural parameters of the cavity distribution.
-            let x_tot = 1.0 / item.fitter.vs(idx);
-            let n_tot = x_tot * item.fitter.ms(idx);
-            let x_cav = x_tot - item.fitter.xs(idx);
-            let n_cav = n_tot - item.fitter.ns(idx);
-
-            self.xs_cav[i] = x_cav;
-            self.ns_cav[i] = n_cav;
-
-            // Adjust the function-space cavity mean & variance.
-            f_mean_cav += coeff * n_cav / x_cav;
-            f_var_cav += coeff * coeff / x_cav;
-        }
-
-        // Moment matching.
-        let (logpart, dlogpart, d2logpart) = self.match_moments(f_mean_cav, f_var_cav);
-
-        for i in 0..self.m {
-            let item = storage.item_mut(self.items[i]);
-            let idx = self.indices[i];
-            let coeff = self.coeffs[i];
-
-            let x_cav = self.xs_cav[i];
-            let n_cav = self.ns_cav[i];
-
-            // Update the elements' parameters.
-            let denom = 1.0 + coeff * coeff * d2logpart / x_cav;
-            let x = -coeff * coeff * d2logpart / denom;
-            let n = coeff * (dlogpart - coeff * (n_cav / x_cav) * d2logpart) / denom;
-
-            *item.fitter.xs_mut(idx) = (1.0 - lr) * item.fitter.xs(idx) + lr * x;
-            *item.fitter.ns_mut(idx) = (1.0 - lr) * item.fitter.ns(idx) + lr * n;
-        }
-
-        let diff = (self.logpart - logpart).abs();
-
-        // Save log partition function value for the log-likelihood.
-        self.logpart = logpart;
-
-        diff
+        self.core.ep_update(lr, storage)
     }
 
-    fn kl_update(&mut self, _lr: f64, _storage: &mut Storage) -> f64 {
-        todo!();
+    fn kl_update(&mut self, lr: f64, storage: &mut Storage) -> f64 {
+        self.core.kl_update(lr, storage)
     }
 }
 
 pub struct LogitWinObservation {
-    //
+    core: Core,
+    _margin: f64,
 }
 
 impl LogitWinObservation {
-    pub fn new(_storage: &mut Storage, _elems: &[(usize, f64)], _t: f64, _margin: f64) -> Self {
-        todo!();
+    pub fn new(storage: &mut Storage, elems: &[(usize, f64)], t: f64, margin: f64) -> Self {
+        LogitWinObservation {
+            core: Core::new(storage, elems, t),
+            _margin: margin,
+        }
     }
 }
 
@@ -139,12 +72,12 @@ impl Observation for LogitWinObservation {
         todo!();
     }
 
-    fn ep_update(&mut self, _lr: f64, _storage: &mut Storage) -> f64 {
-        todo!();
+    fn ep_update(&mut self, lr: f64, storage: &mut Storage) -> f64 {
+        self.core.ep_update(lr, storage)
     }
 
-    fn kl_update(&mut self, _lr: f64, _storage: &mut Storage) -> f64 {
-        todo!();
+    fn kl_update(&mut self, lr: f64, storage: &mut Storage) -> f64 {
+        self.core.kl_update(lr, storage)
     }
 }