how to test the homogeneity of slopes using spss version 25

Since the interaction term was not significant (p=0.977), indicating the covariate parameter estimates are homogenous, you proceeded with an analysis of covariance and found that fertilizer has an effect. We can use equation 1 to calculate the Estimated marginal means as shown on the table.įertilizer 1: 82.750 – 14.750 x 1 – 8.5 x 0 + 4x1.25 = 73 (here fert2=0)įertilizer 2: 82.750 – 14.750 x 0 – 8.5 x 1 + 4x1.25 = 79.25 (here fert1=0)įertilizer 3: 82.750 – 14.750 x 0 – 8.5 x 0 + 4x1.25 = 87.75 (here fert1= and fert2=0)īy specifying an interaction between the covariate (initial height) and factor (fertilizer), you were able to test the homogeneity of the covariate parameter estimates across levels of fertilizer. The coefficient 4 is the average of initial height of the tomato plants.įertilizer 3 (Fert=3) is the reference category. Looking at the Tests of between-subjects effects table shown below, there is some evidence of a fertilizer effect: the F value is 9.294 with a p-value of 0.052.įrom the Parameter estimate table shown below, we can write the following equation: To produce the analysis-of-covariance results, recall the GLM Univariate dialogue box, or from the menus choose:Įxamine and attempt to interpret the output. However, the data is use to illustrate the steps involved in this type of analysis. Yes, the small sample size of the data set. However, is there anything else that you should worry about the data? Now, we can proceed with the analysis knowing that the slopes are equal for different type of fertilizers. We can proceed to estimate the effects of fertilizer type on final height given the initial height. The F value is 0.023 with a p-value of 0.977. The interaction term fert* initial shows no evidence of violation of the equal slopes assumptions.

Does the interaction term fert* initial show any evidence of violation of the equal slopes assumption?

To produce the output, after creating the data file, from the menus choose:Įxamine the output table Test of Between-Subjects Effects and try to interpret it. Using the information on Table 1 above create an SPSS data file. The interaction term provides the test of the null hypothesis of equal slopes. This can be tested by fitting a model containing main effects of fert and initial, as well as the fert* initial interaction. This is known as assumption of equality (homogeneity) of regression slopes. Testing Assumptions: Homogeneity of Regression Slopesīefore we can start the analysis, we need to check the assumption that the regression of final height on initial height is the same for all three types of fertilizer.

How to test the homogeneity of slopes using spss version 25