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# Introduction to SPSS - PowerPoint PPT Presentation

Introduction to SPSS. Data types and SPSS data entry and analysis. In this session. What does SPSS look like? Types of data (revision) Data Entry in SPSS Simple charts in SPSS Summary statistics Contingency tables and crosstabulations Scatterplots and correlations

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### Introduction to SPSS

Data types and SPSS

data entry and analysis

• What does SPSS look like?

• Types of data (revision)

• Data Entry in SPSS

• Simple charts in SPSS

• Summary statistics

• Contingency tables and crosstabulations

• Scatterplots and correlations

• Tests of differences of means

• Menus - Analyse and Charts esp.

• Spreadsheet view of data

• Rows are cases (people, respondents etc.)

• Columns are Variables

• Variable view of data

• Shows detail of each variable type

• We change ticks etc. on a questionnaire into numbers

• One number for each variable for each case

• How we do this depends on the type of variable/data

• Nominal

• Ranked

• Scales/measures

• Mixed types

• Text answers (open ended questions)

• order is arbitrary

• e.g. sex, country of birth, personality type, yes or no.

• Use numeric in SPSS and give value labels.

(e.g. 1=Female, 2=Male, 99=Missing)

(e.g. 1=Yes, 2=No, 99=Missing)

(e.g. 1=UK, 2=Ireland, 3=Pakistan, 4=India, 5=other, 99=Missing)

• in order, 1st, 2nd, 3rd etc.

• e.g. status, social class

• Use numeric in SPSS with value labels

• E.g. 1=Working class, 2=Middle class, 3=Upper class

• E.g. Class of degree, 1=First, 2=Upper second, 3=Lower second, 4=Third, 5=Ordinary, 99=Missing

• Interval - equal units

• e.g. IQ

• Ratio - equal units, zero on scale

• e.g. height, income, family size, age

• Makes sense to say one value is twice another

• Use numeric (or comma, dot or scientific) in SPSS

• E.g. family size, 1, 2, 3, 4 etc.

• E.g. income per year, 25000, 14500, 18650 etc.

• Categorised data

• Actually ranked, but used to identify categories or groups

• e.g. age groups

• = ratio data put into groups

• Use numeric in SPSS and use value labels.

• E.g. Age group, 1=‘Under 18’, 2=‘18-24’, 3=‘25-34’, 4=‘35-44’, 5=‘45-54’, 6=‘55 or greater’

• E.g. answers to open-ended questions

• Either enter text as given (Use String in SPSS)

• Or

• Code or classify answers into one of a small number types. (Use numeric/nominal in SPSS)

• Video by Andy Field

• Used with categorical and ranked variables

• e.g. gender of students taking Health and Illness option

Central Tendency Illness option

• Mean

• = average value

• sum of all the values divided by the number of values

• Mode

• = the most frequent value in a distribution

• (N.B. it is possible to have 2 or more modes, e.g. bimodal distribution)

• Median

• = the half-way value, or the value that divides the ordered distribution in the middle

• The middle score when scores are ordered

• N.B. need to put values into order first

Dispersion and variability Illness option

• Quartiles

• The three values that split the sorted data into four equal parts.

• Second Quartile = median.

• Lower quartile = median of lower half of the data

• Upper quartile = median of upper half of the data

• Need to order the individuals first

• One quarter of the individuals are in each inter-quartile range

Used on Box Plot Illness option

Age of Health and Illness students

Upper quartile

Median

Lower quartile

Variance Illness option

• Average deviation from the mean, squared

• 5.20 is the Sum of Squares

• This depends on number of individuals so we divide by n (5)

• Gives 1.04 which is the variance

Standard Deviation Illness option

• The variance has one problem: it is measured in units squared.

• This isn’t a very meaningful metric so we take the square root value.

• This is the Standard Deviation

Using SPSS Illness option

• Gives mean, median, SD, variance, min, max, range, skew and kurtosis.

• Can also produce stem and leaf, and histogram.

Charts in SPSS Illness option

• Use ‘Chart Builder’ from ‘Graph’ menu or the Legacy menu

• And/or double click chart to edit it.

• E.g. double click to edit bars (e.g. to change from colour to fill pattern).

• Do this in SPSS first before cut and paste to Word

• Label the chart (in SPSS or in Word)

Stem and leaf plots Illness option

• e.g. age of students taking Health and Illness option

• good at showing

• distribution of data

• outliers

• range

Stem and leaf plots e.g. Illness option

Box Plot Illness option

Box Plot Illness option

Fill colour changed.

N.B. numbers refer to case numbers.

Histograms and bar charts Illness option

• Length/height of bar indicates frequency

Histogram Illness option

Fill pattern suitable for black and white printing

Changing the bin size Illness option

Pie chart Illness option

• angle of segment indicates proportion of the whole

Pie Chart Illness option

Shadow and one slice moved out for emphasis

Analysing relationships Illness option

• Contingency tables or crosstabulations

• Compares nominal/categorical variables

• But can include ordinal variables

• N.B. table contains counts (= frequency data)

• One variable on horizontal axis

• One variable on vertical axis

• Row and column total counts known as marginals

Example Illness option

• In the Health and Illness class, are women more likely to be under 21 than men?

Crosstabulations Illness option

• e.g.

• Use column and row percentages to look for relationships

SPSS output Illness option

Chi-square Illness option²

Cross tabulations and Chi-square are tests that can be used to look for a relationship between two variables:

• When the variables are categorical so the data are nominal (or frequency).

• For example, if we wanted to look at the relationship between gender and age.

• There are several different types of Chi-square (²), we will be using the 2 x 2 Chi-square

Another example Illness option

• The Bank employees data

Bank Employees Illness optionChi-Square tests

Chi-Square analysis on SPSS Illness option

• From 6m:30s to 9m:50s

• Low values in cells Illness option

• Get SPSS to output expected values

• Look where these are <5

• Consider recoding to combine cols or rows

Tabulating questionnaire responses Illness option

• Categorical survey data often “collapsed” for purposes of data analysis

An analysis on a sample of 2 (e.g. Black African) would not have been very meaningful!

Recoding variables Illness option

• Ignore t-test for now 6m11s

• Uses old version of SPSS, no submenu now. 6m

• Scatterplots and correlations Illness option

• Looks for association between variables, e.g.

• Population size and GDP

• crime and unemployment rates

• height and weight

• Both variables must be rank, interval or ratio (scale or ordinal in SPSS).

• Thus cannot use variables like, gender, ethnicity, town of birth, occupation.

Scatterplots Illness option

• e.g. age (in years) versus Number of GCSEs

Interpretation Illness option

• As Y increases X increases

• Called correlation

• Regression line model in red

Correlation measures association not causation Illness option

• The older the child the better s/he is at reading

• The less your income the greater the risk of schizophrenia

• Height correlates with weight

• But weight does not cause height

• Height is one of the causes of weight (also body shape, diet, fitness level etc.)

• Numbers of ice creams sold is correlated with the rate of drowning

• Ice creams do not cause drowning (nor vice versa)

• Third variable involved – people swim more and buy more ice creams when it’s warm

• Scatterplot in SPSS Illness option

• Use Graph menu

• http://www.youtube.com/watch?v=UVylQoG4hZM1m50s, ignore polynomial regression

Modifying the Scatterplot Illness option

If mixed data sets Illness option

• Change point icon and/or colour to see different subsets.

• Overall data may have no relationship but subsets might.

• E.g. show male and female respondents.

• Use Chart builder

Correlation Illness option

• Correlation coefficient = measure of strength of relationship, e.g. Pearson’s r

• varies from 0 to 1 with a plus or minus sign

Positive correlation Illness option

• as x increases, y increases

r = 0.7

Negative correlation Illness option

• as x increases, y decreases

r = -0.7

Strong correlation (i.e. close to 1) Illness option

r = 0.9

Weak correlation (i.e. close to 0) Illness option

r = 0.2

Interpretation cont. Illness option

• r2 is a measure of degree of variation in one variable accounted for by variation in the other.

• E.g. If r=0.7 then r2=.49 i.e. just under half the variation is accounted for (rest accounted for by other factors).

• If r=0.3 then r2=0.09 so 91% of the variation is explained by other things.

Significance of r Illness option

• SPSS reports if r is significant at α=0.05

• N.B. this is dependent on sample size to a large extent.

• Other things being equal, larger samples more likely to be significant.

• Usually, size of r is more important than its significance

Pearson Illness option’s r in SPSS

Parametric and non-parametric Illness option

• Some statistics rely on the variables being investigated following a normal distribution. – Called Parametric statistics

• Others can be used if variables are not distributed normally – called Non-parametric statistics.

• Pearson’s r is a parametric statistic

• Kendal’s tau and Spearman’s rho (rank correlation) are non-parametric.

Assessing normality Illness option

• Produce histogram and normal plot

Use statistical test Illness option

• SPSS provides two formal tests for normality : Kolmogorov-Smirnov (K-S) and Shapiro-Wilks (S-W)

• But, there is debate about KS

• Extremely sensitive to departure from normality

• May erroneously imply parametric test not suitable – especially in small sample

• So, always use a histogram as well.

Often can use parametric tests Illness option

• Parametric tests (e.g. Pearson’s r) are robust to departures from normality

• Small, non-normal samples OK

• But use non-parametric if

• Data are skewed (questionnaire data often is)

• Data are bimodal

Spearmans Illness option’s rho

• http://www.youtube.com/watch?v=r_WQe2c-ISU From 4.14 to 4.56

So far… Illness option

• Looked at relationships between nominal variables

• Gender vs age group

• Looked at relationships between scale variables

• Height vs. Weight

• Now combine the two

• Groupsvs a scale variable

• E.g. Gender vs income

• Reminder – IV vs DV Illness option

• IV = independent variable

• What makes a difference, causes effects, is responsible for differences.

• DV = dependent variable

• What is affected by things, what is changed by the IV.

• Gender vs income. Gender = IV, income = DV

• So we investigate the effect of gender on income

Example 1 Illness optionAge group vs. no. of GCSEs

• Using the Health and Illness class data

• Age group defines 2 groups

• Under 21

• 21 and over

• Just two groups

• Can use independent samples t-test

• Independent because the two groups consist of different people.

• t-test compares the means of the 2 groups.

Difference of means Illness option

• Do under 21s have more or fewer GCSEs than 21 and overs?

• Means are different (6.44 & 4.28) but is that significant?

Means are statistically significantly different

Parametric Illness optionvs non-parametric

• Just as in the case of correlations, there are both kinds of tests.

• Need to check if DV is normally distributed.

• Do this visually

• Also use statistical tests

Tests for normality Illness option

• Kolmogorov-Smirnov and Shapiro-Wilk

• If n>50 use KS

• If n≤50 use SW

• Null hypothesis is ‘data are normally distributed’.

• So if p<0.05 then data are significantly different from a normal distribution – use non-parametric tests

• If p≥0.05 then no significant difference – use parametric tests

Checking normality Illness option

• Produce histogram of DV

• Tick box to undertake statistical test

• Interpret results.

t-test Illness option

• Identify your two groups.

• Determine what values in the data indicate those two groups (e.g. 1=female, 2=male)

• Select Analyze:CompareMeans:Independent samples t-test

Mann-Whitney U test Illness option

• Use this when comparing two groups and the DV is not normally distributed

Comparing 3 or more groups Illness option

• ANOVA = Analysis of Variance

• Analyze: Compare Means: One-way ANOVA