A method for data storage includes reading storage values, which represent stored data, from a group of memory cells using read thresholds, and deriving respective soft reliability metrics for the storage values. The storage values are classified into two or more subgroups based on a predefined classification criterion. Independently within each subgroup, a subgroup-specific distribution of the storage values in the subgroup is estimated, and the soft reliability metrics of the storage values in the subgroup are corrected based on the subgroup-specific distribution. The stored data is decoded using the corrected soft reliability metrics.