Nigel Holt and Rob Lewis - Crown House Publishing.

More documents

Recommendations

Info

32 | A2 PSYCHOLOGY: THE STUDENT’S TEXTBOOKNotice that in our data there is one tied score, andonly one negative difference, ranked at positionnumber 4.Step 3: Add up the negative and positive ranks,and choose the smaller of the numbersIn our data, the total sum of the negative ranks is4. The total sum of the positive ranks is a lot more,and comes to 186. The number we want is thesmaller of these, 4. This is your Wilcoxon statistic,referred to as T. It is your observed value.Step 4: Finding the critical valueYou must now compare the observed value to thecritical value in the relevant table. It is included inan appendix at the back of this book. To do this,you need to know:N The number of participants in your task: 20T The observed value you calculated: in this caseit is 4.The type of hypothesisWhether you chose a directional (one- tailed) ornon-directional (two-tailed) hypothesis.Your level of alpha (∝)The level of significance (alpha) you are using.Remember, this is usually 0.05, but may in othercases be much lower.The first thing to do is look down the first columnfor your value of N. Next, read across to therelevant column that refers to your level of alpha.Here you will find the number you are looking for.For simplicity, only the relevant line in the table isreproduced here.Levels of significance for a one-tailed testN 0.05 0.025 0.01 0.00520 60 52 43 26If your value is smaller than the value in the table youcan say with confidence that your results did notoccur by chance. Here, the critical value from thetable is 60, for the significance level of 0.05. Yourvalue (4) is definitely smaller than this.This means that you can say that your results aresignificant. We report this as follows: ‘The differencebetween memory in silence and when listeningto music is significant (p
DATA ANALYSIS AND REPORTING INVESTIGATIONS | 33The descriptive statistics:Mean errorsStandarddeviationMen10.383.74Women6.93.07From the graph and the descriptive statistics itlooks very much as if women make fewer errorswhen doing more than one task at once than domen. We’ll need to do some statistics, in this casea Mann-Whitney, to assess the likelihood thatthese results have occurred by chance.Doing the Mann-Whitney TestStep 1: Rank the dataIn the Mann-Whitney, we rank all the datatogether. By this we mean that the men’s scoresand the women’s scores are all ranked at the sametime. We’ve done this for you here. Notice theusual problem of tied ranks. If you are not sureabout this, look back to the section above.Men(N = 8)1241711812109Rank16318147.51612.510Women(N = 10)Rank9 10584109453125.57.5312.51035.5Step 2: Add up the ranks in each group, callingthem Rank A (R A) and Rank B (R B)Sum of ranks for men (R A)16+3+18+14+7.5+16+12.5+10 = 97Sum of ranks for women (R B)10+5.5+7.5+3+12.5+10+3+5.5+1+16 =74Step 3: Use your values to calculate the resultsof two formulaeFor the Mann-Whitney, we need to calculate twovalues, referred to as U Aand U B. Here are theformulae. They are very similar indeed, so be116careful you are using the right numbers when youcalculate them!1.2.U A=N AN B+U B=N AN B+N A(N A+1)2N B(N B+1)2–R A–R BThese look a little frightening at first, but let’s takeone at a time and see what they look like when weinsert our numbers. As usual, begin by identifyingthe numbers you need and organising yourselfcarefully.U A– the number we are calculatingwith the formulaU B– the number we are calculatingwith the formulaN A= the number in Group A (men) = 8N B= the number in Group B (women) = 10R A= the sum of ranks in Group A (men) = 97R B= the sum of ranks in Group B (women) = 741.U A=N AN B+N A(N A+1)2–R AInserting our values, the formula now looks likethis:U8 (8+1)A=8x10+ –972A little basic mathematics can simplify it furtherto:U8 (9)A=80+ –972This is equal to:U72A=80+ –972which equals:so finally2.U B=N AN B+U A=80+36–97U A=19N B(N B+1)2–R BInserting our values, the formula now looks likethis:10 (10+1)U B=8X10+ –742Extract from A2 Psychology: The Student’s Textbook © Nigel Holt and Rob Lewis ISBN: 9781845901004 www.crownhouse.co.uk
Page 1 and 2: Sample chapter:Psychological Resear
Page 3: the application of scientific mETHO
Page 6 and 7: 6 | A2 PSYCHOLOGY: THE STUDENT’S
Page 8: 8 | A2 PSYCHOLOGY: THE STUDENT’S
Page 11 and 12: DESIGNING PSYCHOLOGICAL INVESTIGATI
Page 19 and 20: DATA ANALYSIS AND REPORTING INVESTI
Page 31: DATA ANALYSIS AND REPORTING INVESTI
Page 41: DATA ANALYSIS AND REPORTING INVESTI

Nigel Holt and Rob Lewis - Crown House Publishing.

Create successful ePaper yourself

Delete template?

Save as template?