A sample should reflect the characteristics of the population of interest to the study - that is, the target population. It is essential to define the target population precisely since failure to do so is likely to lead to the ineffective solving of a research problem.
A population needs to be defined in terms of elements, units and time. The population for a new TV quiz programme, for example, may be defined as:
• males or females
• aged between eight and eighty
• who have watched an existing quiz TV programme in the past 30 days in their own homes.
Elements: males or females, aged between eight and eighty, who have watched an existing quiz programme in the past 30 days Units: household Time: January 1997
The elements that constitute the population are referred to as sampling units. In the new TV quiz programme example, households are designated as the sampling units. It is easier to select households, the place where the programmes are watched, than to interview everyone who qualifies as a member of the population. In some situations, population elements and the sampling unit will be the same, while in others they will be different.
It is up to the marketing researchers to provide explicit instructions to the field workers about the qualifications of the target population. This is achieved with the help of a list of screening questions that can be used to qualify respondents. Screening questions specifically define who should be included in the sample and who should be excluded. Most marketing research surveys exclude certain individuals for a variety of reasons. For example, in the case of a TV sports programme, if a member of a household works for a TV company, then they are excluded. This is usually the first question asked and the interview is terminated at this point if this is the case. These individuals may be excluded for security reasons since they may work for competitors and the researchers would not want them to find out what the study is about.
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