chi square and t-tests, neelam zafar & group
DESCRIPTION
It will help beginners to understand Chi square and T tests in Statistics.TRANSCRIPT
Chi Square and t-testsPresented by
Group C
Introduction
Data analysis can be done basically in three ways using SPSS, asi. Describing data through descriptive
statisticsii. Examining relationships between
variablesiii. Comparing groups to determine
significant differences between them
Introduction
Considering the third category i.e. comparing groups, chi square and t-tests are two of the important tests. These are tests of significance.
Chi-Square Test
Chi-Square test was introduced by Karl Pearson. It follows a specific distribution known as chi-square distribution.
It is used to measure the differences between what is observed and what is expected according to an assumed hypothesis.
Chi-Square Test
The Chi-Square is denoted by X² and the formula is given as:
Here, O = Observed frequencyE = Expected frequency∑ = SummationX²= Chi Square value
A chi-square test is a statistical test commonly used for testing independence and goodness of fit.
Chi-Square Test
Testing independence determines whether two or more observations across two populations are dependent on each other (i.e., whether one variable helps to estimate the other). If the calculated value is less than the table value at certain level of significance for a given degree of freedom, we conclude that null hypotheses stands which means that two attributes are independent or not associated. If calculated value is greater than the table value, we reject the null hypotheses.
Chi-Square Test
This test enables to explain whether or not two attributes are associated. For instance, suppose a study collecting data of survivors in Titanic, for this X² test is useful.
Chi-Square Test
Testing for goodness of fit determines how well the assumed theoretical distribution (such as normal distribution) fit to the observed data. When the calculated value of χ2 is less than the table value at certain level of significance, the fit is considered to be good one and if the calculated value is greater than the table value, the fit is not considered to be good.
Let’s explain its use in SPSS with the help of an example
Chi-Square in SPSSData: Survival on the Titanic by Gender
Analyze →Descriptive Statistics →Crosstabs
Chi-Square in SPSS
Move one variable into ROW and the other into COLUMNS.
CLICK on CELLS
Chi-Square in SPSS
Independent Variable (Gender) is in the Rows
Always show Observed count
Optionally, show Expectedcount
Percentage across the Rows
Click CONTINUEIn main dialogue box,
Click STATISTICS
Chi-Square in SPSS
Choose Chi-Square for hypothesis test
Click Phi and Cramer’s V for measure of strength of the relationship
Click CONTINUEOn main dialogue
box, Click OK
Chi-Square in SPSS
Observed count (yellow highlight)Expected count (orange highlight)Percent within each Gender who Died or Survived
(pink highlight)Report: “Most men on the Titanic (80.2%) died while
most women (71.6%) survived.”
gender * survival Crosstabulation
680.000 168.000 848.000
529.4 318.6 848.0
80.2% 19.8% 100.0%
126.000 317.000 443.000
276.6 166.4 443.0
28.4% 71.6% 100.0%
806.000 485.000 1291.000
806.0 485.0 1291.0
62.4% 37.6% 100.0%
Count
Expected Count
% within gender
Count
Expected Count
% within gender
Count
Expected Count
% within gender
1 Men
2 Women
gender
Total
1 Died 2 Survived
survival
Total
Chi-Square Tests
332.205b 1 .000
330.003 1 .000
335.804 1 .000
.000 .000
331.948 1 .000
1291
Pearson Chi-Square
Continuity Correctiona
Likelihood Ratio
Fisher's Exact Test
Linear-by-LinearAssociation
N of Valid Cases
Value dfAsymp. Sig.
(2-sided)Exact Sig.(2-sided)
Exact Sig.(1-sided)
Computed only for a 2x2 tablea.
0 cells (.0%) have expected count less than 5. The minimum expected count is 166.43.
b.
Chi-Square in SPSS
Pearson chi-square is the default testWhen Sig < alpha, variables are related.Report:
“The relationship is significant (χ2(1) = 332.205, p < .005).”
Symmetric Measures
.507 .000
.507 .000
1291
Phi
Cramer's V
Nominal byNominal
N of Valid Cases
Value Approx. Sig.
Not assuming the null hypothesis.a.
Using the asymptotic standard error assuming the nullhypothesis.
b.
Chi-Square in SPSS
Phi for 2x2 tables Cramer’s V for larger tables
Both range from 0 to 1 with 0 = no relationship
For df = 1◦V = 0.10 is a small
effect◦V = 0.30 is a medium
effect◦V = 0.50 is a large effect
Report: “Genderhad a large effecton chance of survival for the Titanic passengers.”
t-test
The t-test is a basic test that is limited to two groups. For multiple groups, we should have to compare each pair of groups, for example with three groups there would be three tests (AB, AC, BC).
It is used to test whether there is significant difference between the means of two groups, e.g.:
Male v female Full-time v part-time
t-test
There are three types of t-tests as below
A one sample t-test: used when we want to know if there is a significant difference between a sample mean and a test value (known mean from a population or some other value to compare with sample mean), i.e. to compare the mean of a sample with population mean.
t-test
An independent sample t-test: used to compare the mean scores when samples are not matched or for two different groups of subjects i.e. to compare the mean of one sample with the mean of another independent sample.
t-test
Paired sample t-test: used to compare the means of two variables or when samples appear in pairs (e.g. before and after), i.e. to compare between the values (readings) of one sample but in 2 occasions.
Let’s explain performing t-test in SPSS…
t-test in SPSS
Start by clicking “Analyze” on menu barAnalyze → Compare Means → Independent-Samples T-test
Compare Means
Analyze
Independent-Samples T Test
t-test in SPSS
Select the variables to test (Test Variables)
And bring the variables to the “Test Variables” box
Test variables are selected and carried to the box on the right by pressing the arrow
The test variables
t-test in SPSS
Select the grouping variable, i.e. gender; bring it to the “grouping variable” box
Click “Define Groups”
Gender is the grouping variable
t-test in SPSS
Choose “Use specified values”Key in the codes for the variable “gender”
as used in the “Value Labels”. In this case:1 - Male2 - Female
Click “Continue”, then “OK”
Specified values for gender are: 1 (Male) and 2 (Female)
For t-test interpretation lets switch to SPSS
The End