C81MPR Practical Methods (Lab 2) Collecting Data Dr. Danielle Ropar & Dr. Jonathan Stirk

Introduction The aim of this part of the practical is to explain data collection The aim of this part of the practical is to explain data collection There are two sources of data that you should consider There are two sources of data that you should consider –Individual subject data –Group data

Individual subject data When we collect data from a single participant to measure a particular behaviour we can collect: When we collect data from a single participant to measure a particular behaviour we can collect: –One single data point per participant –Several data points per participant

One data point Sometimes it is possible to collect only one data point per subject Sometimes it is possible to collect only one data point per subject For example For example –Height –IQ –Race

Problems with one data point Collecting only one data point is sensible when you are certain that your measurement is accurate Collecting only one data point is sensible when you are certain that your measurement is accurate If there is any reason to believe that your measurement is not always accurate then it is necessary to take several measures If there is any reason to believe that your measurement is not always accurate then it is necessary to take several measures

Several data points per participant Collecting more than one data point per participant is useful because: Collecting more than one data point per participant is useful because: –The estimate of the performance of the participant is less prone to the impact of random variation

The Sternberg example Graph 1

The Sternberg example Graph 2

Group data Usually there is more than one subject in an experiment Usually there is more than one subject in an experiment Collecting data from several subjects has the same advantages as collecting several data points from each subject: Collecting data from several subjects has the same advantages as collecting several data points from each subject: –The estimate of the performance of the participants is less prone to the impact of random variation

How many participants should there be? The number of subjects required in an experiment is dependent upon The number of subjects required in an experiment is dependent upon –The statistical test to be used –The level of significance required –The probability of the alternative hypothesis being true –The size of the effect being examined This information is either gleaned from a search of the literature or based upon the requirements of the researcher This information is either gleaned from a search of the literature or based upon the requirements of the researcher

Number of participants in practicals For the purposes of first year practicals there should be 16 participants per group For the purposes of first year practicals there should be 16 participants per group This number is usually (but not always) big enough to use the appropriate statistical procedures This number is usually (but not always) big enough to use the appropriate statistical procedures This is only a rule of thumb. For real research the proper calculations should be conducted (Using a procedure called a pre- hoc power analysis) This is only a rule of thumb. For real research the proper calculations should be conducted (Using a procedure called a pre- hoc power analysis)

Summary Data is collected from single participants Data is collected from single participants –Whenever possible more than one data point should be collected at each level/condition of the experiment Data is collated from groups of participants Data is collated from groups of participants –There should always be sufficient participants for the particular experiment in mind