hl psychology internal assessment

HL HL Psychology Psychology

Internal Internal AssessmentAssessment

Inferential Statistics

What you should know What you should know after this PowerPoint:after this PowerPoint:

• A concise review of descriptive statistics• Differences between descriptive and inferential

statistics.• Why we use inferential statistics in psychology• How to properly choose an inferential statistics

test.• How to distinguish between various types of data.• How to test for statistical significance.

Descriptive statistics provide Descriptive statistics provide

for..for..• Measure of central tendency

o Gives a typical value for the data seto Tells you where the middle of the data set is

• Measure of dispersiono Indicates how the data are spread outo Tells you what the rest of the data are

Descriptive StatisticsDescriptive Statistics• The aim of descriptive statistics is to give an

accurate summary of the data• The wrong choice of statistic gives a distorted

picture of the data• This can lead to the wrong conclusions being

drawn from the data• Each measure of CT and D has its advantages

and disadvantages

www.psychlotron.org.uk

Measures of Central Measures of Central TendencyTendency

• The mean – total scores divided by the number of scoreso Adv: it uses all the values in the set, so is most sensitive to variations

in the datao Dis: it can be artificially raised or lowered by an extreme value, or by

skewed data

• Use it when the data are normally distributed, unskewed and there are no outliers



• The median – the middle score in a rangeWhat is the median 2,3,3,4,4,4,4,5,5,6,42?

o Adv: it is based on the order of the data, not their actual values, so not distorted by extreme values

o Dis: however, this makes it less sensitive to variations in the data

• Use it when you can’t use the mean because of skew, outliers etc.



• The mode -most frequently occurring valueo Adv: it’s the only measure suitable for summarising category/frequency

datao Dis: for many data sets there is no modal value, or their may be

several

• Use when dealing with frequency data, and/or where there is a clear modal value in the set


Calculate….Calculate….• A psychologist has obtained the following

scores. Answer the questions below.• 8 1 5 5 2 7 1 1 1 4 6

8 9 9 • The range of these scores is

__________________________• The mean of these scores is

__________________________• The mode of these scores is

__________________________• The median is

______________________________________

Measures of Measures of dispersiondispersion

• Range-difference between the smallest and largest value Ex 3,4,7,7,8,9,12,4,17,17,18 =18-3 =16

• Although quick and easy to calculate it is distorted by extreme values

Standard DeviationStandard Deviation• Standard deviation – a measure of the spread

of scores around the mean• It is the most sensitive measure of dispersion

using all available data. It can be used to relate the sample data to the population’s parameters.

SD formulaSD formula• Sum of all participant scores divided by the no of

participants = mean• Subtract the mean from each score• Square each of these scores• Total the squared scores• Divide by one less than the total participants. This

is the variance• Take the square root of the variance.

Work out the SD….Work out the SD….• Scores – 13,6,10,15,10,15,5,9,10,13,6,11,7

GraphsGraphs• Bar chart –Shows data for categories that the

researcher is interested in comparing

HistogramHistogram• Shows data for all categories even those with

zero value

Frequency Frequency polygon/line graphpolygon/line graph

• Shows two sets of data on one graph

Pie chartsPie charts• Show the proportion of all scores gained by

various categories

Inferential Inferential StatisticsStatistics

HL IA ONLY

Inferential StatisticsInferential Statistics• With inferential statistics, you are trying to reach

conclusions that extend beyond the immediate data alone. For instance, we use inferential statistics to try to infer from the sample data what the population might think.

• Or, we use inferential statistics to make judgments of the probability that an observed difference between groups is a significant one or one that might have happened by chance in this study.

Inferential StatisticsInferential Statistics• Thus, we use inferential statistics to

make inferences from our data to more general conditions; we use descriptive statistics simply to describe what's going on in our data.

What you are bring What you are bring asked to do (HL IA).asked to do (HL IA).

• An appropriate inferential statistical test has been chosen and explicitly justified. Results of the inferential test is accurately stated.

• The null hypothesis has been accepted or rejected according to the results of the statistical test. A statement of statistical significance is appropriate and clear.

What you are bring What you are bring asked to do (HL IA).asked to do (HL IA).

• The information you have obtained from participants takes the form of raw data. This should go into the appendices, and you should use your results to calculate descriptive statistics appropriate to your to data.

• The test you choose is dependent on the level of measurement of your data and whether you used independent samples or repeated measures.

Levels of Levels of MeasurementMeasurement

• Nominal-frequency headcount; things can only belong to one category ex the no of students wearing yellow shirts.

• Ordinal –data which is ranked or put in order. It is not known what the interval between each rank is ex 1st,2nd,3rd time in a swimming trial

• Interval/ratio- measurement on a scale where the intervals are known and equal (ratio has a true zero point; interval can move into negs. Ex of ratio is time in secs.

Levels of data: Levels of data: nominalnominal

• Which newspaper paper do you read regularly?

• We can put these into categories.

Levels of Data: ordinalLevels of Data: ordinal• What grade did you get for each of your portfolio?

• These can be put in order… highest to lowest

Levels of data: intervalLevels of data: interval• How quick is your reaction time?

• We can measure and compare the exact time because the intervals on the ruler are equal.

Inferential testsInferential tests• Provide a calculated value based on the results of

the investigation• This value is then compared to a critical value

(statistical tables) to determine if the results are significant

• In chi square, sign test, spearman’s rho the calculated value must exceed the critical value.

Choosing an Choosing an inferential testinferential test

• Nominal data and independent measures design = Chi square test

• Ordinal data and independent measures design = Mann Whitney U

• Interval and ratio data and independent measures design = Unrelated T-test

• Nominal data and repeated measures design =Sign test• Ordinal data and repeated measures design = Wilcoxon

test• Interval or ratio data and repeated measures design =

related T-test• More info:

http://hs-psychology-ibhl.ism-online.org/files/2011/09/Choosing-an-inferential.pdf

A directional hypothesisA directional hypothesis• Very often, we state before we test the hypothesis

in which direction of the results will fall. Our hypothesis is usually directional (meaning we are predicting an increase or decrease in a time or score)and the appropriate statistical test of the hypothesis is called one-tailed.

• Once you have collected the data. Decide which test you need to administer. Only one person in your group needs to work out the mathematics.

Using Tests of Significance – The Using Tests of Significance – The

General procedureGeneral procedure

• Choose appropriate statistical test• Calculate statistical test• Compare the test with the critical values. These

can be found in the back of the Research methods text book, or mathematics statistic books, or online.

• Decide which side of the critical value your result is on.

• Report the decision.

Inferential statistics- Inferential statistics-

indicating how significant indicating how significant

results are.results are.• A significant result is one where there is a low

probability that chance factors were responsible for observed difference

• 5% level of significance, in psychology, is acceptable (P is less than 0.05)

• There is less than a 5 % likelihood that the difference was due to chance.

Key Terms you will need to Key Terms you will need to

look up and define.look up and define.Critical valueDegrees of freedomP value/levelSignificanceOne-Tailed TestTwo-Tailed TestType 1 errorType 2 errorInterval OrdinalNominal

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