SSI POV
Understanding Incidence
Incidence in market research is the rate of occurrence of a stated event within a given population.
For example we may want to look at how many people in the US own a home. We may also want to
know how many of those homeowners bought a refrigerator in the past month. There are two ways to
refer to incidence in market research: Survey Incidence and the incidence of something asked within the
survey, generally pertaining to a question or set of questions. We will refer to the latter as Question
Incidence.
Survey Incidence is the rate at which people qualify for a survey. This is derived from the total number
of completes divided by the total number of completes + screenouts. In most cases, this is the
“incidence” that your sample provider is referring to when they check feasibility on a project.
The Question Incidence refers to the occurrence of a stated event based on everyone who answered
that question. So going back to the example above, let’s say we are running a study looking for
respondents that own a home and bought a refrigerator in the past month. We know the Survey
Incidence based on the screening criteria, but we want to know the incidence of home ownership. To
calculate this, we would look at the homeownership and calculate how many people said they owned a
home regardless of whether or not they qualified for the full study.
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[email protected] | surveysampling.com
Incidence rates while seemingly simple quickly become complicated based on study parameters and
survey logic. To illustrate this, suppose we are also putting quotas on the brands of refrigerators
purchased. In this example, if the respondent owns a home, purchased a refrigerator in the past month
and purchased a brand where the quota hasn’t been reached, they will complete the study. However if
they purchased a brand whose quota is full they will be removed from the survey and marked as a
“quota full.” How does this affect the Question Incidence? We can see in the chart below.
In this example, the Survey Incidence is 40% since there were only 400 respondents that made it
through the full questionnaire to be counted as a complete.
So far this seems simple enough. But what is the incidence of respondents who purchased a Brand A
refrigerator in the past month? We might say 11% (100 Brand A completes (100) divided by the total
number of completes (400) + screenouts (500). We know that 500 people screened out by Q2
(purchased a refrigerator in the past month) and that we have 400 completes total for study.
This is a common mistake made. Here’s what happened. We are counting every person that screened
out of the survey (500) but we are only counting those who completed the study after forced quotas
were put in place. Therefore we are not counting everyone who qualified for the study.
Once we count those people that qualified, we find an incidence of 20% for those that purchased a
Brand A refrigerator in the past month (200 Brand A purchasers/1000 survey starts).
©2017 Survey Sampling International, LLC
[email protected] | surveysampling.com
When calculating the Question Incidence, we need to be sure we are including everyone that was asked
that particular question. This is especially true when there are forced quotas around a parameter that is
“related” to the incidence in question, such as in the example above.
Contact your SSI representative for assistance in correctly measuring incidence.
©2017 Survey Sampling International, LLC
[email protected] | surveysampling.com