Evidence-appraisal glossary
Last observation carried forward
Last observation carried forward (LOCF) is a simple imputation method that replaces a participant's missing later value with their most recent measured one.
Also called: LOCF, carry-forward imputation.
LOCF was once common because it is easy and keeps every randomized participant in the analysis, but it assumes a person's condition stayed frozen after they left, which is rarely true. In a disease that worsens over time it can flatter a treatment by pretending dropouts held steady, and in an improving condition it can understate benefit, so it is not the safe or conservative choice it was long assumed to be. Modern guidance generally prefers multiple imputation or mixed models, so seeing LOCF is a cue to check how much it drove the result.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.