Survey MethodologyData interpretation and presentation

EPID 626

Lecture 11

References

• Many elements of this lecture were taken from– Fink, Arlene. How to Report on Surveys.

Sage Publications. 1995.– Babbie, Earl. Survey Research Methods.

Wadsworth Publishing Company. 1990.

Analytic modes• Univariate analysis

– Examination of the distribution of cases on only one variable at a time

• Aim is description• Example: SBP distribution among subjects

• Bivariate analysis– Examination of the distribution of cases on one

dependent and one independent variable• Aim is explanation• Example: SBP distribution by sex

Analytic modes (2)

• Multivariate analysis– Examination of the distribution of cases on

one dependent and more than one independent variable

• Aim is explanation• Example: distribution of SBP by race and sex

Presenting univariate data

Options (example=age):

• List all respondents with their age

• Tabulate the number of respondents within categories of age

• Use measurements of central tendency and dispersion– Appropriate for continuous data

Measures of central tendency

• Mean– Average

• Mode– The most frequent attribute

• Median– The middle attribute in a ranked distribution

of observed attributes

Measures of dispersion• Range

– Distance between the lowest and highest values

– Often presented as min and max• Example

– range= 40– or range= 20-60

• Standard deviation– Distribution of observations about the mean

Measures of dispersion (2)

• Quartile deviation or semi-interquartile range– One-half of the distance between the

bottom of the first quartile and the top of the fourth quartile

Constructing bivariate tables

• Column headings are determined by the most important comparison– cases vs. controls, men vs. women etc.

• There are many options for format; decision should be made based on ease of interpretation

Guidelines for formatting tables

• Headings or titles should sufficiently describe what is in the table

• The original content of the variable (the survey question) should be presented in the table or in accompanying text

• Values or categories of each variable should be indicated

Guidelines for formatting tables (2)

• When presenting percentages, the base upon which they are computed should be indicated

• Number of respondents with missing data should be indicated

• If appropriate, statistical values (frequencies or percents) should be in descending order

ExampleFrequency of reported behavior (N=5670)

Risk behavior SAQ ACASI Binge drink 1789 1943

Drive without a seatbelt 789 537

Unprotected sex 352 476

Guidelines for formatting tables (3)

• Use a standardized set of symbols to call the reader’s attention to key aspects of the table– definitions

– statistical significance

– different denominator for %

• P-value format– use “=“ for exact p-value

– use “<“ for other p-values

Presenting data

“First you tell’em what you’re gonna tell’em, then you tell’em, then you tell’em what you told’em, and then you tell’em what to do with it.”

-Preacher’s proverb

Presenting data

• A reader should be able to replicate your study

• Data without methods is meaningless

• A reader should be able to verify all percents and measures of association such as OR or RR

Interpretation of results

• Interpretation should follow the flow from your research question and study objectives

• Interpretation should be guided by statistical significance as appropriate