Chapter 2 – Experimental Design and Data Collection

Math 22

Introductory Statistics

Error Due to Bias and Chance

• Bias - A systematic tendency to misrepresent the population.

• The object of any experimental design is to eliminate bias and reduce chance error as much as possible.

Types of Design

• Confounding factor (lurking variable) – a hidden factor that has an effect on the response we are attempting to measure.

• Experimental (treatment) Group – those persons or objects that receive the treatment of interest in an experimental design.

• Control Group – those persons or objects that do not receive the treatment of interest. Rather, they may receive the “old” treatment or a placebo.

Statistical Designs

• Experiments - Attempts to determine a cause and effect relationship between two or more variables.

• Blind Experiment – the test subjects do not know if they are getting the experimental treatment or the placebo.

• Double Blind Experiment – neither the test subject nor the experimenter measuring the response knows to which group the test subjects have been assigned (treatment or placebo).

Statistical Designs

• Prospective Study – Study of future events.• Randomization – An excellent way to reduce

bias.• Retrospective Study - Study of past events.• Cross Sectional Study – Study of events at the

current time (one point in time). Data represents what is going on at a certain cross section of time.

Collecting Data

To obtain reliable information that will help answer your research questions, follow these steps:

• Determine the objectives of the study you are undertaking.

• Define the population of interest.• Choose the variables that you will measure

in the study.

Collecting Data

• Decide on an appropriate design for producing data.

• Collect the data.

• Determine the appropriate descriptive and/or inferential data analysis techniques.

Types of Random Sampling

• Simple Random Sample - To select the sample in such a way that every sample of that size has the same chance of being chosen.

• Systematic Random Sample

• Stratified Random Sample

Surveys

A properly designed survey reports the following information:

• A description of the sampled population

• A description of the method of contact for interviews

• The response rate

• The exact wording of the questions

Surveys

• The timing of the interview.

• The size of the sample (or the margin of error)

• The sampling technique