Research Methods in Psychology Sampling and Experimental Design

Research Methods in Psychology

Sampling and Experimental Designs

Friday, 27 January 2012

Lesson 12: Research Methods Variables and Hypothesising

Exam Question:

Read the following research question and respond to the following

Does drinking alcohol effect reaction time?a) What is the dependent variable? (1 mark)

b) What is the independent variable? (1 mark)c) What is a possible extraneous variable? (1 mark)d) Write an operational hypothesis for this research

question. (3 marks)


Model response:

a) DV: Reaction time

b) IV: Alcohol Consumption

c) Many possible responses e.g. age, gender, sleep deprivation, strength of eye sight, natural skill at task

d) It was hypothesised that Victorian adults aged 20-30 who drink 3, 375ml bottles of beer, 20 minutes prior to taking the

“Reaction Speed Simulator” test will produce a lower reaction time (lower percentage score) than those who did

not consume alcohol.


Lesson 13: Research Methods: Sampling, Participant Selection and Experimental Designs

OUTCOMES:

Define population Define sample

Describe the process of sampling procedures including random, stratified and random stratified

Describe the process of participate allocation to groups (experimental and control) including random allocationDiscuss the advantages and disadvantages of different

experimental designs including repeated measures, matched participants and independent groups

Describe the placebo effect and ways of managing its occurrence

Describe the experimenter effect and ways of managing its occurrence


SamplingSampling is the selection of participants for a research.

Population refers to the group which the research wishes to draw conclusions from.

The term sample refers to the members of the population that have been chosen to take part in the research.

Sampling procedures must ensure that the sample is representative of the population.


Representative Samples

Two techniques are used to ensure a representative sample: 1)Random Sampling 2)Stratified Sampling and Stratified Random Sampling.


Random SamplingA sampling procedure in which every member of the population has an equal chance of being selected

Examples include: 1) Picking a name out of a hat 2) Tattslotto 3) Closing my eyes and selecting a number to match that number with student id numbers.


Stratified Sampling and Stratified Random Sampling

Is used when you wish to eliminate the effects of confounding variables.

The effects of a certain variable can be eliminated as a possible confounding variable in an experiment.

The variable could be any personal attribute, such as age, years of education, ethnicity, gender, IQ etc.

Involves six procedures:


1) Identifying a property that we believe may interfere with the effects of the IV on the value of the DV. 2) Measuring that property for each member of the population. 3) Dividing the population into particular strata (groups) based on the value of that variable. 4) Deciding on the number of participants required for the experiment.5) Selecting participants in the same proportions as exist in the population to make up the sample (stratified sample).6) Selecting a random sample from each stratum, in the same proportions as exist in the population (stratified random sample).


Stratified Sample


Which to use?

Sophisticated, advanced Psychological research studies use Stratified Sampling, however it is very time consuming and expensive, therefore majority of research uses random sampling.

More so common, as the name suggests, Psychological research uses a sample of convenience, which although is biased is quick, easy and cheap!


SAMPLING FLOW CHART


Participant Allocation: Experimental & Control GroupsThe experimental method uses two different groups called the experimental and control groups.

The experimental group are exposed to the IV, known as the ‘treatment’.

The control group do not receive the treatment (IV).

The purpose of the experimental group is to show the effects of the IV on the value of the DV.

The purpose of the control group is to form a basis for comparison with the experimental group.


Experimental and Control Group Allocation

It is super important that all participants have an equal chance of being in the experimental or control group. That is Random Allocation.

When there is a large enough sample, both the experimental and control groups will be equivalent on all participant characteristics therefore the presence or absence of the IV is the only difference between them.

E.G. If we had all males in the experimental group and all females in the control group, then an obvious extraneous variable will be gender.


Experimental Designs

There are three popular experimental designs

Repeated Measures Design

Matched Participants Design

Independent Groups Design


Repeated Measures Design (within participants design)

In a repeated measures design participants experience both the experimental and control groups.

This is possible by conducting the experiment on two occasions and then comparing the two results.


ADVANTAGES: 1)Using the same participants means that confounding variables that are participant depend are eliminated. 2)Allows for fewer participants to be used than with other designs.

DISADVANTAGES: 1) Time consuming - drop outs 2) Confounding variables such as Order Effects: a) Participants may perform better on the task when doing for a second time (practise effect). b) Participants may do worse the second time because of fatigue or boredom.


Counterbalancing

Used to overcome order effect.

In counterbalancing, half the participants will first perform the task with the IV present (experimental condition) and then perform the task with the IV absent (control condition). The other half of the participants will experience the conditions in the reverse order.


Matched Participants Design

Enables the researcher to identify a variable that is likely confound and to eliminate the effects of this variable from the experiment.

Participants can be ranked in accordance with their scores on this variable and then allocated to the respective groups.


E.G. A sports coach developed a new game plan that would help the team reach the playoffs. He decided to test this by giving the experimental group the instruction but not the control group. Because individual skills would be a confounding variable, he decided to ‘match’ the groups. The two highest skilled players will be randomly allocated to either the experimental or control group, the third and fourth most skilled will then be randomly allocated to either and so on and so forth until all players were allocated to a group resulting in the same mean skill percentage in both groups.


Advantages: The variable on which the participants are ‘matched’ will not influence the results because its effects will be the same in the experimental and control groups.

Disadvantages: It is very time consuming (and therefore expensive) to find out the value of this variable for every participant. Also, if one of the pair drops out, the scores for the other must also be eliminated.


Allocates participants to the experimental or control group at random

Independent Groups Design (between participants design)


Advantages: The independent groups design can be done at once and drop-outs are unlikely.

Disadvantages: The procedure needs a large number of participants to ensure that the spread of participant variables in the sample will match the spread in the population.


PLACEBO EFFECT

Participants expectations

This may effect the DV resulting in invalid

results

Can be eliminated by using single blind procedure, that is participants are unaware of which group

they are in.


EXPERIMENTER EFFECT

Experimenter influences experiment

For example: Experimenter treats participants differently

depending on the group they are in which in turn influences the

behaviour of the participant and effects the results

Can be eliminated by using a double blind procedure, that is, neither the experimenter or the participants are aware of

whether they are in the experimental or control group


Research Methods in Psychology Sampling and Experimental Design

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