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Identifying variables and issues in experimental design

Lisa is a university tutor who is interested in studying the use of mobile phones and their possible adverse effect on people's ability to notice and respond to visual stimuli. The results from such research could have important implications for driving behaviour (Horrey and Wickens, 2006).

She devised a task where participants had to monitor a rapidly changing visual display on a computer screen for the appearance of some target pictorial stimuli. At any one time a participant saw a picture of one object on the screen which appeared for just ½ sec. After a 100 msec gap a new picture appeared. Participants were told to watch out for five different pictured objects and to press a button as quickly as possible every time one of them appeared. In a pilot test it was found that the target pictures were all easily identifiable under normal viewing conditions.

Lisa recruited participants from two university cohorts: 20 undergraduates from University of Wales, Bangor where she taught during the day (average age 21 years) and 12 3rd-year students from her own tutorial group (average age 37 years). Participants were tested individually, completing Session 1 mid-morning and session 2 after a 30-minute break for coffee. During Session 1 there were no distractions and the participant observed the computer display for ten minutes. During this time 30 target stimuli appeared and Lisa recorded the time taken to respond to those detected and the total number that were responded to. Out of interest she also recorded 'false positive' responses (pressing the button when a target stimulus was not present). During Session 2 the same sequence was presented but now participants had their mobile phone in hand and a caller, as arranged by the experimenter, called them at random times and asked them questions about their mobile phone, their age, their interests and their reasons for taking a psychology course. They were required to answer the phone every time it rang and to engage in conversation with the caller and reply to the questions asked. With their free hand they also had to respond to the visual target stimuli on the computer screen as in Session 1.

For each question choose the answer(s) that seems most appropriate.

(a) In this experiment the independent variable was (choose one only):

(i) The University to which the participant belonged.
(ii) The types of questions that were asked during Session 2.
(iii) The type of session (with or without mobile phone).
(iv) The time the testing took place (before or after coffee break).
(v) The age of the participant.
(vi) The number of times participants 'responded' in each session when no target was present.

(b) There were two main dependent variables in the study. Identify them in the list below (choose two only).

(i) The number of target stimuli detected in total across the two sessions.
(ii) The average time taken to respond to target stimuli in each session.
(iii) The answers given to the questions asked on the telephone during Session 2. (iv) The average age of the participants.
(v) How many target stimuli were detected in each session.
(vi) The number of target stimuli that appeared on the screen.

(c) The design of the experiment was (choose one only):

(i) within participants, because all the participants were shown the same sequence of stimuli.
(ii) within participants, because the number of target stimuli they were looking for was the same for all of them (five).
(iii) within participants, because all the participants did Sessions 1 and 2.
(iv) between participants because there were two different sessions.
(v) between participants because participants came from two different Universities. (vi) between participants because they did Sessions 1 and 2 at different times of day.

(d) The researcher controlled for two of the following things in her study (choose two only):

(i) The number of target stimuli that appeared during each session.
(ii) The length of time spent on the telephone by the participant during Session 2.
(iii) How recognizable the pictures for the target stimuli were.
(iv) The average age in each group.
(v) The hand they used to press the button.

(e) Unfortunately the study was flawed because of the presence of two potentially confounding variables. Identify them from the following list (choose two only):

(i) practice on the particular sequence of stimuli
(ii) the age of the participant
(iii) the University at which the participant was studying
(iv) whether a mobile phone had to be answered during the session or not
(v) time of day of the session
(vi) the nature of the questions asked by the caller.

(f) For the two answers you chose for the previous question, describe briefly what changes you would make to the design to eliminate the confounding effects.

(g) When Lisa came to look at the data afterwards she found there were no real performance differences between the two sessions, which was quite surprising. Suggest some possible reasons for this.

Solution Preview

(a) In this experiment the independent variable was:

(iii) The type of session (with or without mobile phone).

The independent variable (IV) is what you're manipulating to see how it affects performance. Although there are a LOT of things changed between the sessions in this experiment, the one that Lisa is interested in is whether having a mobile phone influences performance.

(b) There were two main dependent variables in the study. Identify them in the list below.

(ii) The average time taken to respond to target stimuli in each session.
(v) How many target stimuli were detected in each session.

The dependent variable (DV) is the variable you're measuring. In this experiment, participants are told to watch for items and press a button as quickly as possible when they see them. This implies that Lisa is looking not only at the number of target stimuli that people detect, but also how fast they are at responding to them. Remember, the DV performance will be broken down across the 2 sessions, since we're interested in comparing performance with the cell phone and without.

(c) The design of the experiment was

(iii) within participants, because all the participants did Sessions 1 and 2.

When we talk about a within-participants design, we're talking about a situation where the participants do both levels of the IV (which in ...

Solution Summary

This solution explains how to identify independent and dependent variables using an example. It also discusses flaws in experimental design and how to fix those, control of confounds, and the experimental design.

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