Sally Akarolo-Anthony MD MSc Research Fellow, Doctoral Candidate

Harvard School of Public Health

Summer 2012

Part I Types of errors Importance of correcting for errors

Part II Methods to correct for measurement errors

We will cover Part I today. We will cover Part II after lectures on Correlation, ANOVA and Linear Regression.

Types of Errors

Specific sources are innumerable

They can be thought as 2 general types

Random Systematic

Random error Law of large numbers applies The average value of many repeated

measures approaches the true value.

Systematic error The mean of repeated errors does not

approach the true value

In epidemiologic studies, random or systematic errors, or both can occur at two different levels: within a person and between persons

Random within-person error Random between-person error Systematic within-person Systematic between-person error

Random within-person error Fluctuation in dietary intake Errors in measurement of intake on any one

day Usually not important when considering their

effects on epidemiologic studies.

Systematic within-person error FFQ: Omission or misinterpretation of

important food item for a subject Open ended dietary assessment: deny or

exaggerate

Most literature in epidemiology addressing the issue of measurement error assume the within person error is strictly random. This is probably due to: 1) Statistical theory mainly based on assumption of random error. 2) Systematic error is considerably more difficult to measure.

Random within-person error can be measured with a replicate measure for the same sample of subjects. Reproducibility study

Systematic within-person error requires a second, superior measure of exposure. Validation study

In many aspects of epidemiology, there is no perfect gold standard of exposure ascertainment

Random between-person error May result from using only one or a few replicate

measures per subject in the presence of within-person error

May be a consequence of systematic within-person

errors that are randomly distributed among subjects. Implies an overestimation for some subjects is

counterbalanced by an underestimation for others.

The mean for a large group of subjects is the true mean for the group.

The SD for the group is exaggerated.

Systematic between-person error Results from systematic within-person

errors that are nonrandomly distributed among subjects.

The mean for a large group of subjects is thus incorrect.

The SD for the group is correct.

However, if individuals are affected to various degrees, the SD estimated will not represent the true SD.

Systematic between-person error Are likely to be frequent and have many causes:

Omission of a commonly eaten food from a

standardized questionnaire

Use of an incorrect nutrient composition value

More commonly, Random and systematic errors are likely to exist in combination.

Walter Willett, Correction for measurement error 1998

Focus of epidemiology is on associations with disease, therefore the impact of exposure measurement error on measures of association is important.

In general, random within/between-person error tends to decrease correlation and regression coefficients toward 0 and bias relative risks toward 1.

Systematic errors that affect all persons equally, do not affect measures of association.

Youre conducting a study in a population like Nigeria, where abortions are illegal and everyone in your control group underreports induced abortion rates? The options below are not mutually exclusive a) Systematic error [within/between] b) Random error [between/within persons]

Correction of Measurement Error

Methods

Walter Willett 1998 Correction for the effects of measurement error Nutritional Epidemiology 2nd Edition

Correction of Measurement ErrorThis topic will be covered in 2 partsPART ITypes of errorSlide Number 5Slide Number 6Within-person errorSlide Number 8Slide Number 9Slide Number 10Slide Number 11Between-person errorSlide Number 13Slide Number 14Slide Number 15Slide Number 16Slide Number 17Slide Number 18Slide Number 19Exercise: What kind of error(s) can you have:PART IIAcknowledgement