10. Cuando de utiliza el test exacto de Fisher | Curso de SPSS

What is Fisher's Exact Test?

Overview of Fisher's Exact Test

• The Fisher's Exact Test is a probability test used when both variables in the analysis are fixed, meaning their final distributions cannot be altered.

Example Scenario

• An example involves two varieties of beans (variety one and variety two) exposed to a type of weevil to determine if there is a preference for one bean variety over the other.

Contingency Table Construction

• A contingency table is created with beans in rows and larvae in columns, showing 18 beans from variety one and 16 from variety two, totaling 34 beans and six larvae.

Fixed Marginals Explained

• The initial setup ensures that the outcome will always reflect the starting quantities: 18 beans of variety one and 16 of variety two. This characteristic defines them as fixed marginals.

Larvae Distribution

• The six larvae can affect either or both bean varieties, but the total number affected will always remain at six, reinforcing that these marginal values are also fixed.

Application of Fisher's Exact Test

Statistical Analysis Process

• In experiments where both marginals are fixed, only the internal distribution changes; thus, Fisher's Exact Test should be applied. Most experiments involve smaller sample sizes like this case with 34 observations.

Chi-Square Selection in Software

• When analyzing data using statistical software, selecting chi-square not only computes chi-square statistics but also provides additional tests including Fisher’s exact statistic.

Interpretation of Results

• The output includes p-values indicating significance; here it shows that the p-value is not significant (greater than 0.05), suggesting no preference by weevils for either bean variety based on this test.

Conclusion on Preference Testing

• Using Fisher’s Exact Test indicates that there is no significant preference among weevils for either bean type due to fixed marginals being a key characteristic in this study.