---
title: Left censored data
description: "Censored data (see [entry](https://www.psyctc.org/psyctc/glossary2/censored-data/)) is data where at least some values are not know accurately but are known to be bigger than a given value, less than a given value, or between two values. Left censored data is the first…"
url: "https://www.psyctc.org/psyctc/glossary2/left-censored-data/"
updated: "2024-12-15"
category: All OM book glossary entries
---

# Left censored data

Censored data (see [entry](https://www.psyctc.org/psyctc/glossary2/censored-data/)) is data where at least some values are not know accurately but are known to be bigger than a given value, less than a given value, or between two values.

Left censored data is the first of those: data where at least some values are known to be below a value but not known precisely. This typically happens in routine database data or when looking retrospectively at events. We may know that one client's first contact with a service was on or before some date when a routine database was created but it may not be possible to find out the precise first date. This is the complement of right censored data.

#### Details

This matters because if we ignore the censoring we have introduced a bias into our estimation of general durations of therapies for that study. However, there are statistical methods that handle censoring and will avoid that bias (given some sensible assumptions mostly that the pattern of duration hasn't actually changed across the period of the study). See survival analysis for probably the biggest class of such methods.

#### Try also

[Censored data](https://www.psyctc.org/psyctc/glossary2/censored-data/)
[Frailty analysis](https://www.psyctc.org/psyctc/glossary2/frailty-analysis-models/)
[Interval censoring](https://www.psyctc.org/psyctc/glossary2/interval-censored-data/)
[Right censoring](https://www.psyctc.org/psyctc/glossary2/right-censored-data/)
[Survival analysis](https://www.psyctc.org/psyctc/glossary2/survival-analysis/)
[Type I censoring](https://www.psyctc.org/psyctc/glossary2/type-i-censoring/)
[Type II censoring](https://www.psyctc.org/psyctc/glossary2/type-ii-censored-data/)

#### Chapters

Not covered in the OMbook.

#### Online resources

None forseeable.

#### Dates

First created 10.xii.24, links tweaked 15.xii.24.