Statistics in SPSS Lecture 3 Petr Soukup, Charles University in Prague

Descriptive stat II 2

Real data for lectures ISSP 1999 Short info about data 3

Descriptive stat / review LECTURE 1 Central tendency: ? Dispersion = variance Skewness Kurtosis TODAY’S LECTURE 4

Dispersion/Variance Measure the size of difference between individual values (card. variable) The well-known measure = variance (formula and logic by picture) For real data analysis better to use standard deviation (why?) Also range can be used (=MAX-MIN) 5

Variance in SPSS More procedures, use more groups for comparison (DATA-SPLIT FILE) ANALYZE-DESCRIPTIVE STATISTICS-DESCRIPTIVES ANALYZE-DESCRIPTIVE STATISTICS-FREQUENCIES ANALYZE-DESCRIPTIVE STATISTICS-EXPLORE NEW 6

Skewness Measure the shape of the distribution (card. variable), ?symmetric or not? Skewed to the left or right (pictures) Values for skewness 7

Kurtosis Measure the „peak“ of the distribution (card. variable), ?big or small? Platykurtic – low kurtosis or flat Leptokurtic – low kurtosis or big peak 8

Skewness and kurtosis in SPSS ANALYZE-DESCRIPTIVE STATISTICS-EXPLORE Comparison of two groups Standardized normal distribution as a benchmark 9

Box plot as special tool Show many descriptive statistics in one picture Very easy to describe your data Example in SPSS: ANALYZE-DESCRIPTIVE STATISTICS-EXPLORE-PLOTS Possible to compare groups 10

Missing values 11

Missing values System missing values (NO INFO) User missing value (USER DEFINES) Examples in SPSS Impact of definition on descriptive statistics (income variable) Always remember: Check your data and define missings if necessary (otherwise GIGO) 12

Data transformation (Creation of a new variable) 13

Data transformation Use current variable(s) for creation of new one(s) Many procedures Most common: RECODE COMPUTE COUNT 14

RECODE Change coding scheme of the variable Can be saved in current variable or into new one Examples in SPSS (gender, educational level) 15

COMPUTE Compute new variable by mathematical formula (use current variable(s)) Examples in SPSS (age) 16

COUNT Compute occurence of specific value(s) in the set of variables Examples in SPSS 17

HW 18

HW3 Try to describe one cardinal variable / i.e. mean, median, standard deviation, skewness and kurtosis. Compute and compare results for male and female. 19

Thanks for your attention 20