Defining the Population
A sample in a market research study is drawn from a population (sometimes called a universe). For example, when conducting political polls, our population is the people that can vote in the next election. When conducting studies of fast moving consumer goods (e.g., breakfast cereal, laundry detergent), our population is grocery buyers.
Defining the population for a study involves two separate decisions. The first decision is what unit to study. The unit is the 'what' that is to be counted. Commonly-used units include people, consumers, users, households, main grocery buyers, businesses, business decision makers and voters.
The second decision is to define the boundaries of the population: which units are included and which are excluded. Most obviously this is done via geography, where a decision is made about which countries (or states within countries) should be included in the research. Typically boundaries are also defined based on behavior, such as consumption of a particular product. Examples of population definitions for studies include:
- Main Grocery Buyers that have purchased instant coffee in the past four weeks in Australia.
- People that will be eligible to vote in the next US Presidential Election.
The classic mistake
A common mistake when defining population is to over-define the population such that it is too small. One chip manufacturer undertook a study where the market was defined as “Males aged 25 to 34 that have consumed premium chips in the past two weeks”. When it was suggested that the market was too narrow to be sensible, the brand manager (male in late 20s) indicated that this was not a problem as the age group was “aspirational”. The reason that such a narrowly defined market is problematic is that inevitably most of the people who are really in the market for the product are not represented by the study and thus the study likely becomes biased.
There are some simple rules of thumb that can be used to see if a population has been too narrowly defined:
- If the biggest selling products in the market are getting more than 20% of their sales from people outside the defined market, the definition is likely too narrow.
- If the definition involves any demographic variables (e.g., age, gender, income), it is probably a Target Market rather than a sensible population definition, unless the demographics are causally related to aspects of people's physiology or life circumstances (e.g., if the study is about retirement, then age is causally related to this, whereas if the study is about chip buying, age may be correlated but is not causally related).
- The definition must be operational. For example, rather than “women capable of bearing children” a more operational definition is “women aged 12 to 50”.