Experimental DesignSection 1.3

Designing a Statistical Study

1. Identify the variable(s) of interest (the focus) and the population of the study.

2. Develop a detailed plan for collecting data. Make sure sample is part of the population.

3. Collect the Data4. Describe the data, using descriptive statistic

techniques.5. Interpret the data and make decisions about

the population using inferential statistics.6. Identify any possible errors.

Ways to Collect Data

1. Observational Study› Researcher observes and measures but does not

change the environment at all. Example: Researches observed and recorded what

children up to three years old did with nonfood objects (saw if they put it in their mouths)

2. Experiment› Treatment applied to part of a population and

responses are observed. You can also use a control group and a placebo. Example: Diabetics take a pill to see if helps reduce

their risk of heart disease while a control group took a water pill.

Ways to Collect Data

3. Simulation› Mathematical or physical model used to

reproduce the conditions of a situation › Done when experiment is too dangerous or costly.

Example: Automobiles use dummies when they are studying the effects of crashes on humans.

4. Survey› Investigation of one or more characteristics of a

population (interview, mail, telephone) Example: A survey conducted on females

physicians to determine whether the primary reason for their career choice is financial stability.

Examples:

1. A study of the effect of changing flight patterns on the number of airplane accidents.› Simulation

2. A study of the effect of eating oatmeal on lowering blood pressure.› Experiment

3. A study of how fourth grade students solve a puzzle› Observation

4. A study of U.S. residents’ approval rating of U.S. president› Survey

Three Key Elements of a well designed experiment are:

1.) Control influential factors› A confounding variable occurs when an

experimental cannot tell the difference between the effects of different factors on a variable.

Example: A coffee shop owner wants to attract more customers into her shop so she decorates it in bright colors. At the same time a new shopping mall opens up. If the business at the shopping mall increases you can not determine if it is the new colors or the shopping mall.

Three Key Elements of a well designed experiment are:› Placebo Effect occurs when a subject

acting favorable to a placebo even when they received no medication.

Example: Someone who has depression is given medicine which in fact is a water pill. The person then starts to feel better because they believe the medicine is working.

Three Key Elements of a well designed experiment are:

2.) Randomization – Randomly assign subjects to different treatment groups› Could have groups being completely random.› Could have groups be in blocks

Blocks are groups of subjects have the same characteristics

› Could have groups be in a randomized block design Example: An experiment of a weight loss drink. You

may create blocks of 20-29 year olds, 30-39, and 40-49. Then in those blocks randomly pick people to be in the treatment group or control group.

Three Key Elements of a well designed experiment are:

3.) Replacement› The repetition of an experiment using a

large group of subjects.› HAVE LARGE SAMPLE SIZES

Placebo Effect

Placebo – a faux treatment that looks like the real treatment (i.e. sugar pill). It acts as a control.

Placebo Effect – occurs when an untreated subject incorrectly believes that he/she is receiving a treatment and reports an improvement in symptoms.

Example:

The company identifies ten adults who are heavy smokers. Five of the subjects are given the new gum and the other five subjects are given a placebo. After two months, the subjects are evaluated and it is found that the five subjects using the new gum have quit smoking.› Sample size too small, should be replicated.› Results of the 5 adults who were given the

placebo are not given.

Example:

The company identifies 1,000 adults who are heavy smokers. The subjects are divided into blocks according to their gender. Females are given the new gum and males are given the placebo. After 2 months, the female group has a significant number of subjects who have quit smoking.› Groups not similar. Divide into blocks and then

split the blocks into treatment group and control group.

› Don’t know the results of the men's group

Sampling Techniques

Sampling Techniques

Census› A count or measure of an entire

population (costly and difficult) Sampling

› A count or measure of part of a population

Sampling error› The difference between the results of a

sample and those of a population

5 Sampling Techniques

1. Simple Random Sample› Every possible sample of the same size

has the chance of being selected› Appendix B› Assign a different number to every member of

the population and use of a random number generator to choose group

5 Sampling Techniques

2. Stratified Sample› Used when it is important to have members

from each segment of the population in our sample

› Members of a population are divided into two or more subsets that are called strata that share a similar characteristic such as age, gender, ethnicity, etc.

› A sample is randomly selected from each strata

› Example: Divide homes into socioeconomic levels

5 Sampling Techniques

3. Cluster Sample› Use when population falls into naturally

occurring subgroups, each having similar characteristics

› Divide population into groups called clusters

› Select all members in one or more clusters (not all)

› Example: Divide into zip codes, Class courses

5 Sampling Techniques

4. Systematic Sample› Each member of the population is assigned a number

› Members are ordered in some way

› Starting number is selected, and then sample members are selected at regular intervals (every 3rd, every 5th, etc.)

› Example: Assign numbers to each house in Cranberry Township and then select every 100th household.

5 Sampling Technique

5. Convenience Sample› Only use the available members of the

population› Not recommended!!

You get biased results

Example

You select a class at random and question each student in the class.› Cluster

You divide the student population with respect to majors and randomly select and question some students in each major.› Stratified

You question every 20th student you see in the hall.› Systematic

You assign each student a number and generate random numbers. You then question each student whose number is randomly selected.› Simple Random Sample

Homework

Page 25: 2, 3, 17-27