Lesson 5 DATA ANALYSIS. Am I using and independent groups design or repeated measures? Independent groups Mann- Whitney U test Repeated measures Wilcoxon.

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Presentation transcript:

Lesson 5 DATA ANALYSIS

Am I using and independent groups design or repeated measures? Independent groups Mann- Whitney U test Repeated measures Wilcoxon test Experiments

Our raw data for today: Ppt No of words recalled Method Method Independent groups or repeated measures?

Wilcoxon  Used to check for statistical significance when it’s a test of difference for related data. Repeated measures only.

Step 1  The difference between each participants’ two scores is calculated, you also need to state if it’s a positive or negative number Ppt No of words recalled Method Method Difference Sign (+/-)

Step 2  The differences are given a rank to show their order – the lowest gets rank 1. Ignore +/- signs; and 0. If there are multiple scores which are the same, calculate their mean rank Ppt No of words recalled Method Method Difference Rank Sign (+/-) Mean rank: We add up the ranks that these scores would have had, if they had been different from each other (the ranks of 1, 2,3 and 4), and then divide the sum of these ( = 10) by the number of ranks involved (4). This gives us an "average" rank,

Step 3  Total the ranks for the positive differences and for the negative differences. The smallest is the observed value (T) Ppt No of words recalled Method Method Difference Rank Sign (+/-)

Step 3  Total the ranks for the positive differences and for the negative differences. The smallest is the observed value (T)  Positive =  Negative =  So, the observed value of T is=  The observed value needs to be equal to or less than the critical value to be significant

Step 4  Compare your observed value to the critical value to find out whether your results are statistically significant (and if you can reject the null hypothesis!)  Is it directional or non-directional  Does that mean it’s one-tailed or two-tailed?  What level of significance does Psychology use?  How many participants did we have in our experiment? Ignore scores of 0! N – the number of scores left ignoring those with those with 0 difference.

Step 4  Compare your observed value to the critical value to find out whether your results are statistically significant (and if you can reject the null hypothesis!) Level of significance for two-tailed test Level o21f significance for one-tailed test N

Step 4  Compare your observed value to the critical value to find out whether your results are statistically significant (and if you can reject the null hypothesis!) Level of significance for two-tailed test Level of significance for one-tailed test N

Step 4  Observed value = 8.5  Critical value = 3  WILCOXON = Observed value equal to or less than Critical value to be significant

Step 5 How to write this up:  The results are not significant  suggesting there is no difference between method 1 and method 2 in terms of memory improvement.  The observed value (8.5) is greater than the critical value (3)  at p<0.05, one-tailed, N=7.  Therefore, we have to accept the null hypothesis, and reject the alternate hypothesis.

New set of a data Participant no.Condition ACondition BDifferenceRankSign +/- N=

H/W  Make up a one tailed hypothesis  Make up a set of data (10 participants)  Work out the observed and critical value  Explain whether you would accept your hypothesis  Check you answer using the excel spreadsheet or calculator on line: 