Marketing research Determining the size

Marketing research
Part 12 – determining the size of a sample
Sample size refers to the number of respondents in a survey rather than how these respondents are selected.
But before we begin to study this chapter, we have to understand 2 things:
There is no relationship between the size of the sample and its representativeness of the population from which it is drawn.
The size of the sample affects the sample accuracy of results.
But if your sample size increased twice, it doesn’t mean, that the research is 2 times more accurate!

Marketing research
Part 12 – determining the size of a sample
The Confidence Interval method of
determining sample size

Marketing research
Part 12 – determining the size of a sample
Sample size and accuracy
Nonsampling error – all sources of error other than sample selection method size
Sampling error – is the difference between the sample finding and the true population value due to the fact that a sample was taken.

Marketing research
Part 12 – determining the size of a sample
2. The larger the size of the (probability) sample, the less is its sample error.
The relationship between sample size and sample error

Marketing research
Part 12 – determining the size of a sample
3. p and q: The notion of Variability
Variability is defined as the amount of dissimilarity (or similarity) in respondent’s answers to a particular question.
For example, we may find that question: “the next time you order pizza, will you use Domino’s?”

Marketing research
Part 12 – determining the size of a sample
Graph of variability 50/50 – 90/10

Marketing research
Part 12 – determining the size of a sample
4. The notion of a confidence interval

Sample error is less with larger sample sizes

Marketing research
Part 12 – determining the size of a sample
5. The accuracy of a probability sample is independent of the size of the population.
6. Accuracy desired from the estimate
it is used to indicate how close your sample percentage finding will be to the true population percentage if you were to report the study many, many times.

Marketing research
Part 12 – determining the size of a sample
(p.27. Question 9)
Last year, Lipton Tea Company conducted a mall-intercept study at six regional malls around the country and found that 20% of the public preferred tea over coffee as a midafternoon hot drink.
This year, Lipton wants to have a nationwide telephone survey performed with random digit dialing. What sample size should be used in this year’s study in order to achieve an accuracy level of ± 2.5% at the 99% level of confidence? What about at the 95% level of confidence?
Case study

Marketing research
Part 12 – determining the size of a sample