Evidence-appraisal glossary
Missing data
Missing data are values a study intended to collect but did not obtain, such as outcomes for participants who dropped out or skipped a visit.
Also called: missingness, incomplete data.
Missing data threaten a trial's conclusions because the people with absent values often differ from those with complete records, so ignoring them can bias the result. Analysts distinguish data missing at random from data whose absence is related to the outcome itself, which is harder to correct. Watch for how much data were missing and how the authors handled it, since a large or lopsided gap can matter more than the headline effect.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.