Methods to control extraneous variables
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Methods to Control Extraneous Variables. Content Covered Placebo effects Experimenter effects Experiment designs Statistics Inferential statistics Ethical considerations in psychological research. Study Design Content.

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Methods to Control Extraneous Variables

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Methods to control extraneous variables

Methods to Control Extraneous Variables

Content Covered

Placebo effects

Experimenter effects

Experiment designs


Inferential statistics

Ethical considerations in psychological research

Study design content

Study Design Content

  • order effects, experimenter effect, placebo effects; ways of minimising confounding and extraneous variables including type of experiment, counterbalancing, single and double blind procedures, placebos; evaluation of different types of experimental research designs including independent-groups, matched-participants, repeated-measures; reporting conventions

  • statistics: measures of central tendency including mean, median and mode; interpretation of p-values and conclusions; reliability including internal consistency; validity including construct and external; evaluation of research in terms of generalising the findings to the population

Study design content1

Study Design Content

• ethical principles and professional conduct: the role of the experimenter; protection and security of participants’ rights; confidentiality; voluntary participation; withdrawal rights; informed consent procedures; use of deception in research; debriefing; use of animals in research; role of ethics committees.

Placebo effects

Placebo Effects

  • In psychological research a placebo effect may occur when a participants response (the DV) is influenced not be the IV, but rather than by the expectancy of the participant of how they are expected to behave

  • For example…

  • If a study was examining a new drug and we had an experimental group who was receiving the drug in a tablet and a control group who was not, the participants who know they are receiving the treatment may have effects based upon their expectancy of what they think should happen

  • Box 6 – The Hawthorne Effect, pg. 21

Placebo effects1

Placebo Effects

  • To counter this the experimenter may use a placebo

  • A placebo is a fake treatment which is used to reduce participant expectancy in a treatment. It could be in the form of a sugar pill so that both the experimental and control groups are unsure as to what group they are in

  • This is known as a single-blind procedure

Experimenter effects

Experimenter Effects

  • Even though we may have minimised participant effects via single-blind procedure, there still could exist experimenter bias

  • Experimenter bias is where the experimenter may have expectancy about the results based upon the treatment

Experimenter effects1

Experimenter Effects

  • To overcome this extraneous variable, a double-blind procedure may be used

  • A double blind procedure is where neither the participants nor the researcher conducting the experiment, knows which participants are in experimental or control group

  • A third person is used to oversee the placement of the participants into the groups

  • Learning Activity 7 – Review Questions, pg. 23

Experimental design

Experimental Design

  • The experimental design is another way to minimise the effects of extraneous variables

  • There are three main experimental designs we can use in psychological research

  • They are Repeated measures, Matched participants and Independent Groups

Repeated measures


  • In a repeated-measures design, one group of participants undertakes both experimental conditions (the experimental condition and the control condition)

Fig 1.18 pg. 28

Repeated measures1


  • Advantages

  • Subject variables are kept highly controlled as they are consistent between conditions

  • Fewer participants are required

  • Disadvantages

  • Repetition effects may occur – participants may become bored or fatigued after the first condition

  • The practice effect could make the results invalid – participants may learn how to improve in the second condition from the first condition they undertake



  • To overcome the effects of repetition, the experimenter may use counterbalancing

  • Counterbalancing involves placing half the participants in the experimental group and half in the control group first, thereby balancing the effects of the order

Matched participants


  • A matched-participants design involves placing equivalent pairs of participants into each group

  • Participants in each condition are paired according to any important variables, which if left uncontrolled, may confound the results – could include gender, age, intelligence

Fig 1.19 pg. 29

Matched participants1


  • Advantages

  • Eliminates order effects such as fatigue as the participants only undertake one condition

  • Disadvantages

  • Although it attempts to keep subject characteristics constant, participants can never be perfectly matched

  • The process of matching participants is time-consuming

  • If one participant leaves the experiment the matched participant in the other group must have their results exluded

Independent groups design

Independent-Groups Design

  • The independent-groups design using random allocation to assign participants to one of the control or experimental group

  • Each participant only undertakes the condition they are assigned to – either the control or experimental group

Fig 1.20 pg. 30

Independent groups design1

Independent-Groups Design

  • Advantages

  • Easier to select participants

  • Also eliminates order effects such as fatigue

  • Disadvantages

  • Subject variables could occur despite random allocation

  • Participants only undertake one condition

  • The least effective in minimising extraneous variables

Experimental design1

Experimental Design

  • Copy Figures 1.18, 1.19, and 1.20 into your notes

  • Activity 9 – Research Methods, Minimising extraneous variables



  • Quantitative statistics take the form of numerical values such as the weight of a participant

  • Qualitative statistics are factual descriptions about the characteristics of a subjects behaviour and is usually in the form of words



  • There are three measures of central tendency

    • The mean – the average of a set of scores, the most sensitive measure of central tendency but can be distorted by extreme high or low values

    • The median – the middle number in a set of scores

    • The mode – the most commonly occurring number

Inferential statistics

Inferential Statistics

  • Inferential statistics are formal data analyses that measure the likelihood of results obtained for a study occurring by chance

  • Measures of statistical significance indicate whether or not the results obtained in an experiment are due to chance, rather than the manipulation of the IV

  • Significance refers to the results being important, if the results are significant then they important in terms of supporting the hypothesis

P value


  • The p-value is an inferential statistic used to represent the probability level for deciding whether chance factors are responsible for the results obtained

  • If the results are obtained by chance then our hypothesis cannot be supported

P value1


  • A significant result is where it has been determined there is a low probability that the results of a research study were due to chance

  • A p-value of p<0.05 occurs when there is probability of chance occurring 5 or fewer times in 100 repetitions of the research

  • In psychology a p<0.05 is accepted to mean that the results are statistically significant and are not due to chance, but rather due to the influence of the IV

  • We can also say that the hypothesis is supported if p<0.05

P value2


  • p-values with stricter conditions are also used when the researcher wants to be really sure of their results (a final drug trial before human subjects are used perhaps?)

  • For example p<0.01(chance less than 1 in 100) or p<0.001(chance less than 1 in 1000)

  • Learning Activity 15 – Review Questions

Conclusions and generalisations

Conclusions and Generalisations

  • A conclusion is a decision or judgement made about the results from an investigation may mean

  • A generalisation is a decision or judgement made about how the results could be applied to other members of the population which is being studied

Ethical considerations in psychological research

Ethical Considerations in Psychological Research

  • The term ethics refers to standards that guide individuals to acceptable or desirable conduct

  • Ethics are present in many different fields and organisations (Schools, law etc.)

  • The “National Statement on Ethical Conduct in Research involving Humans” is a code of ethics which provides guidelines which must be followed when working with people in research situations and all other areas of professional practice

Participants rights

Participants’ Rights

  • Confidentiality

  • Voluntary Participation

  • Informed Consent Procedures

  • Withdrawal Rights

  • Deception in Research

  • Debriefing



  • Refers to a participants right to privacy, so that details of their involvement in the study cannot be revealed in a manner which enables individuals to be identified, unless written consent in obtained

  • This can refer to access of data from the research as well as storage and disposal of research data

Voluntary participation

Voluntary Participation

  • The researcher must ensure that a participants participation in a research study is entirely voluntary and that no pressure is placed on them to take part

Informed consent procedures

Informed Consent Procedures

  • Where possible participants must be informed of the nature and purpose of the research

  • This informed consent needs to be documented and is usually done in the form of a consent form

Informed consent procedures1

Informed Consent Procedures

  • Where research may involve participants in situations of physical or mental stress, the researcher must inform them of the procedures to be used and the effects that can be expected

  • If research involves participants in high levels of emotional arousal, the researcher must ensure no psychologically vulnerable person participates

  • Where an individual is unable to give informed consent (such as a child or intellectually disabled individual) the researcher must obtain appropriate consent from those legally responsible for the individual

Withdrawal rights

Withdrawal Rights

  • The researcher must inform the participant that they are free to participate or decline to participate or withdraw at any time

  • The participant must be free to withdraw from the research at any time, without giving a reason

Deception in research

Deception in Research

  • Sometimes, giving a participant too much information about a study may influence their behaviour during the study, therefore influencing the results

  • When it is necessary to conduct a study without fully informing the participants of its true purpose prior to the study, the researcher must ensure that participants do not suffer distress and are fully debriefed at the end of the study



  • It is important that the researcher debriefs the participant in regards to the research and results following the conclusion of the research

  • Learning Activity 19 – Identifying ethical issues, pg.54

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