Add in replacement and a warning

bilby/gw/prior.py

@@ -53,8 +53,6 @@ def convert_to_flat_in_component_mass_prior(result, fraction=0.25):
     priors = result.priors
     posterior = result.posterior
 
-    logger.info("Resampling posterior to flat-in-component mass")
-
     priors["chirp_mass"] = Constraint(
         priors["chirp_mass"].minimum, priors["chirp_mass"].maximum,
         "chirp_mass", latex_label=priors["chirp_mass"].latex_label)
@@ -69,7 +67,18 @@ def convert_to_flat_in_component_mass_prior(result, fraction=0.25):
         latex_label=priors["mass_2"].latex_label)
 
     weights = posterior.mass_1 ** 2 / posterior.chirp_mass
-    result.posterior = posterior.sample(frac=fraction, weights=weights)
+
+    logger.info("Resampling posterior to flat-in-component mass")
+    effective_sample_size = sum(weights)**2 / sum(weights**2)
+    n_posterior = len(posterior)
+    if fraction > effective_sample_size / n_posterior:
+        logger.warning(
+            "Sampling posterior of length {} with fraction {}, but "
+            "effective_sample_size / len(posterior) = {}. This may produce "
+            "biased results"
+            .format(n_posterior, fraction, effective_sample_size / n_posterior)
+        )
+    result.posterior = posterior.sample(frac=fraction, weights=weights, replacement=True)
     result.meta_data["reweighted_to_flat_in_component_mass"] = True
     return result