1. Read
A sample survey
A survey can be conducted by personal interviews, by telephone, by mail or by email. The personal interview technique has the advantage of having a high response rate and a high quality of answers obtained. However, it is the most expensive technique. A survey conducted by mail or email is the least expensive method but the response rate is usually very low.
Sample surveys can be used in
market researching
quality controls
opinion polls
society researching
There is always an uncertainty of the results, because of the sample. The reliability can be increased by a larger sample an also by using better measuring instruments.
A simple random sample
A simple random sample is a sample that is selected in such a way that each member of the population has the same chance of being included in the sample.
2. Open the Wordlink about Central limit theorem, confidence interval, margin error below and read it carefully.
3. Examples.
Ex1. A food company is planning to market a new kind of cereal. However, before marketing the product, the company wants to find what percentage of people will like it. The company's research department selected a random sample of 500 persons and asked them to taste this cereal. Of these 500 persons, 290 said they liked it.
a) Find, with a 95% confidence level, what percentage of all people will like this cereal, i.e. construct a 95% confidence interval.
b) What is the point estimate (i.e. the value of a sample statistic that is used to estimate a population parameter) of the population proportion? What is the margin of error of this estimate?
Ex2. A product is claimed to have a market share of 20%. You want to do a market research of market share. Of what size should the sample be when having a margin error of at most +/- 2% points with 95% confidence?
The answers to Ex1 and Ex2 are in an Excellink below.
4. Read
The size of the sample
If the sample is a random sample, it should be representative for the population. There could be influenced random factors, but they get even when the sample is big enough. The larger size of the sample, the better it representates the population.
The size of the sample is conclusive for how well you can examine your results. For example, the opinion of the americans can be examined by the same size of the sample as the opinion of the finns.
A sample of 1000 ( or 1000-2000) people gives usually a reliable result.
5.For variable of scale type: How to construct a confidence interval for the mean in SPSS:
Analyze, Descriptive Statistics, Explore, choose variable into Dependent list, change confidence level in Statistics (95% predefined), OK.
Solved example as link below.
For a variable of nominal type: To construct a confidence interval for a proportion you have to use the formulas and calculate in Excel.
6. Exercise: Construct confidence intervals and margin error for two variables in your own survey. Use confidence level of 99%.