What is a Survey ?

What is a Survey ?

A Survey collects information

Is a Census from all the population

Is a Poll if for political information

Is a Sample Survey if from just a sample of a population

Conditions for a Survey

• Answer the Objectives• Be Unbiased, Accurate• Be Generalizable• Be Ethical• Be Economical

Before a Survey

• Define the Questions to be answer.• Define the Sampling strategy.• Design and Test the Questionnaire.• Train the field workers.• Define the technique for crossvalidation.• Define the final Analysis.

During the Survey

- Verify and crossvalidate the Questionnaire

- Check Time table and Budget

After the Survey- Crosscheck again all the data

- Perform the main analysis

- Perform any exploratory data analysis.

The Question

The first task is to clearly and concisely define the Main Question of interest as well as the Target Population of the study.

Sampling UnitThe individualA familyA group, i.e. a class.

Sampling FrameExisting listing

New listing

Define the Precision or Tolerated error of the estimates : 1% , 5%.

Are there any Nuisance factor to be accounted for, which ones are important?

Stratification

• Sample size proportional to strata size

• Equal sample sizes from all strata

• Sample size inversely proportional to strata variability.

Multiple Steps Sampling

Big Units2nd Units

SamplingUnits

Questionnaire

• Short and Concise

• Open Questions

• Multiple choice

• Yes/No

Internal Validity

How to check if the subject answers truthfully?

- Ask the same question twice?Marital statusSpouse age

- Check frequency versus known data

External validity

Are the appropriate questions asked?

Are the questions understandable by all the subjects in the sample.

Study of Trends

Cohort

Panel

same same

samesame

newnew

Interviewers in Follow Up Studies

• Preferably be the same.

• If not the training must be the same.• They must be comparable.

Preparing for Validation

• Lists of valid responses for each question.

• Define code for: Missing values.

• Not applicable.

• Do not know.

• Automatically set the ‘Not applicable’ based on some previous question.

Definition of the Analysis• Specific Objective 1

- Test to answer specific objective 1.

• Specific Objective 2 - Test to answer specific objective 2.

• Secondary objectives - Test to investigate secondary objectives.

• Exploratory Data Analysis.

During the Survey

• Collect questionnaires daily.

• Check yesterday questionnaires for missing or invalid answers.

• Re-interview invalid questionnaires

The Time TableThe often forgotten tasks.

• Is the number of answered questionnaires according to the predicted for this date?.

• Is more time needed to fill up the required number of questionnaires?

• Are more interviewers needed to complete the task in time?

The Interim View• Crosstabulate the descriptor variables. i.e. age,

gender, occupation, etc.• Are they similar to the known ( or assumed)

distribution in the population?• Are we getting a representative or a biased

sample?.• Do we need more advertising of the study in some

sectors of the population?

After the Survey

• Check for outliers in all variables, both singly and in logical pairs.

• Are the distributions and scattergrams plausible ?

• Decide whether to impute or to eliminate the clear mistakes.

Statistical AnalysisMain Analysis

Description of the sample Predefined Main Statistical analysisStatistical significance of results

Exploratory AnalysisDescribe test / procedure usedReport results

The Report• List of Objectives.

• Objectives achieved.

• Sample size estimated.

• Response rate in the sample.

• Main Statistical Analysis.

• Results with significance or Confidence Interval of estimators.

• Statistical Power of the tests performed.

• Results of the Exploratory Data Analysis

Conclusions• What was achieved.• Did the main Analysis agree with

predictions and if not why?• Interpretation of the results of the main

Analysis.• Were any new hypothesis generated by

the EDA.• Interpretation of the findings by the

EDA.• Ideas for new research.