Experimental Design

Learning Objectives: To develop an understanding of experimental design. Success Criteria: 1) Take part in a discussion about IVs and DVs. 2) Answer a past paper question on IVs and DVs. 3) Complete page 10 of your booklet on experimental design.

Research Methods To conduct an experiment you need… IV & DV Hypotheses (directional & non-directional) Controls Operationalisation

IV and DV Independent variables – the thing you change This variable is manipulated by the researcher and is assumed to effect the DV Dependent variables – the thing you measure This variable is measured by the researcher and is affected by the independent variable

Discussion activity Which is the independent variable and dependent variable for each of the following research questions... Does the weather affect mood? Do boys and girls like different music? Do we lose our memory as we get older? Can a person be conditioned to fear spiders? Is there less attention in classes on a Friday afternoon? Is there such a thing as a born killer?

Task Identify the IV and DV from each of the examples on page 9 of your booklet.

Past Exam Question AO3 = 2 marks IV    The interview, type of interview, method of interview, ‘standard interview or / and cognitive interview’, whether or not cognitive interview. DV   Number of items recalled, recall, what they remembered. A psychologist used an independent groups design to investigate whether or not a cognitive interview was more effective than a standard interview, in recalling information. For this experiment, participants were recruited from an advertisement placed in a local paper. The advertisement informed the participants that they would be watching a film of a violent crime and that they would be interviewed about the content by a male police officer. The psychologist compared the mean number of items recalled in the cognitive interview with the mean number recalled in the standard interview. Identify the independent variable and the dependent variable in this experiment. (2 marks)

Experimental Design In a lab experiment we manipulate an IV There are usually two values of the IV e.g. Noise or no noise Chocolate or no chocolate These determine the conditions (groups) of the experiment These groups can be arranged in several different ways

Experimental Design There are three techniques psychologists can use to design their experiment: Independent groups design Repeated measures design Matched pairs design See page 10 of your booklet

Independent Groups Design Recruit a group of participants Divide them into two This group does the experimental task with the IV set for condition 1 This group does the experimental task with the IV set for condition 2 Measure the DV for each group Compare the results for the two groups

Participant Variables Variation between participants can affect DV Could mask an effect (false negative) Could imply an effect where none exists (false positive) This is a problem with independent measures You can control this by random assignment to groups, or Use repeated measures or matched participants instead

Repeated Measures Design Recruit a group of participants Condition 1 Condition 2 The group does the experimental task with the IV set for condition 1 The group repeats the experimental task with the IV set for condition 2 Compare the results for the two conditions

Counterbalancing This is a really important control when using repeated measures Reduces ‘carry over’ effects (when you learn from first attempt and apply it to the second)…so… Half participants do condition A then B Other half do condition B then A Fully counterbalanced: ABBA

Fatigue, Boredom & Practice Carrying out a task repeatedly leads to changes in performance Deterioration as participants become tired or bored, or Improvement due to practice This is a problem with repeated measures, so Leave a long gap between conditions, or Counterbalance the design, or Use independent measures or matched participants

Matched Pairs Design Compare the results for the matched pairs Recruit a group of participants Find out what sorts of people you have in the group Recruit another group that matches them one for one Treat the experiment as independent measures Condition 1 Condition 2 Compare the results for the matched pairs

Attrition When participants drop out of a study (attrition) data is lost Fewer data = less powerful study This can be a problem with matched participants (and repeated measures if there is a gap between conditions) Loss of a participant in these designs means losing their data from both conditions

In a nut shell... Experimental Design What it means Independent measures design One group does one thing, and one does another Repeated measures design We all do one thing, then we all do the other thing Matched pairs design It’s independent measures, but the groups are deliberately balanced

Independent Groups Design Which experimental design has been used in this famous study? Was it a good choice? Loftus & Palmer (1974) took 45 students from a Californian university. They all watched a film clip of two cars colliding. Then they were split into 5 groups of 9. Each group was asked a question – “About how fast were the cars going when they hit/smashed/bumped/collided/contacted each other?” Estimates of speed were found to vary according to which verb had been used in the question. Independent Groups Design

Repeated measures Design Which experimental design has been used in this famous study? Was it a good choice? Kiecolt-Glaser (1984) took blood samples from 75 first year medical students a) one month before their final examinations (relatively low stress), and (b) during the examinations (high stress). She found that they had reduced immune systems during times of stress. Repeated measures Design

Independent Groups Design Which experimental design has been used in this famous study? Was it a good choice? Nisbett & Wilson divided students into 2 groups. They watched 2 different videos of the same lecturer. In one he gave advice to students in a warm and friendly manner; in the other video he said exactly the same words but in a cold and aloof way. Students were asked to score him on likeability. The ones who had seen the warm and friendly video scored the lecturer higher but they didn’t know why! Independent Groups Design

Repeated measures Design Which experimental design has been used in this famous study? Was it a good choice?  Stroop (1935) gave 70 college undergraduates a sheet of paper to read where the words matched the colour of the ink in which they were printed (e.g. ‘pink’ was printed in pink ink). The participants were asked to name the ink colour. They were also asked to read a sheet where the words did not match the ink colour (e.g. ‘pink’ would be printed in blue ink). Participants were asked to name the ink colour. He found that responses were slower when ink and word did not match. Counterbalancing was used to eliminate order effects. Repeated measures Design

Independent Groups Design Which experimental design has been used in this famous study? Was it a good choice? Bandura, Ross and Ross (1961) tested 74 preschoolers. They were put into groups: a control group; single sex groups who saw a violent model of either same sex or a different sex; and single-sex groups who saw a non-violent model of either the same sex or a different sex. Children who saw a model who was violent were more likely to imitate violent acts, especially if the model was the same sex as them. Independent Groups Design

Tasks On page 10 of your booklet is a grid. You need to complete this. Give a brief description of each type of experimental design. Give details of the advantages and disadvantages of each method. The textbooks on the shelf will help. Extension: Complete the questions at the bottom of page 10 onto your own paper.