Re: Follow-up to piecewise regression question

Posted by Ryan on
URL: http://spssx-discussion.165.s1.nabble.com/Follow-up-to-piecewise-regression-question-tp5668949p5681386.html

Carol,

 

Something's wrong with your analysis. Your "Estimates of Fixed Effects" should include three main effect terms (representing intercepts) and three interaction terms (representing slopes). Instead, it looks like you have a reference category for age. Did you run the analysis a couple different ways?

 

Ryan

On Wed, May 2, 2012 at 2:34 PM, Parise, Carol A. <[hidden email]> wrote:

 

Here is the result of using age in its original form and the \TEST statements.

 

 

What just dawned on me as i was looking at this is that since there is a significant interaction for age3*age, the main effect of age3 is no longer applicable - just like in any other anova.

 

The efffect of age on time, depends on which part of the age continum one is on.  So, there is no effect of being under age 38 on finish time but being between 38-51 means you get slower by .20 hrs for every year. If you are over 50, then you still slow down but not as much.

 

 

Estimates of Fixed Effectsa

Parameter

Estimate

Std. Error

df

t

Sig.

95% Confidence Interval

						Lower Bound	Upper Bound
[age3=1.00]	.171610	1.293038	13,805	.133	.894	-2.362920	2.706140
[age3=2.00]	-6.124152	1.282973	13,805	-4.773	.000	-8.638953	-3.609351
[age3=3.00]	0b	0	.	.	.	.	.
[age3=1.00] * NewAge	.036716	.022293	13,805.000	1.647	.100	-.006981	.080413
[age3=2.00] * NewAge	.203924	.016284	13,805	12.523	.000	.172004	.235843
[age3=3.00] * NewAge	.082119	.019039	13,805	4.313	.000	.044800	.119438

 

 

/TEST = "diff in slopes between  <38 and 38-50" age3*newAge 1 -1  0

 

The estimate is simply 0.037-0.203 and it's significant which means that the slopes change from one age group to another. I believe that this provides the rationale for selecting this cutpoint in the data.

Contrast Estimatesa,b

Contrast

Estimate

Std. Error

df

Test Value

t

Sig.

95% Confidence Interval

							Lower Bound	Upper Bound
L1	-.167208	.027493	13,805	0	-6.082	.000	-.221098	-.113318

a. diff in slopes between  <38 and 38-50

 

 

 

 

  /TEST = "diff in slopes between  <38 and 51+" age3*newAge 1  0 -1

The estimate is  0.203-0.082 and it's not significant which means that the slopes aren't really that different.

 

Contrast

Estimate

Std. Error

df

Test Value

t

Sig.

95% Confidence Interval

							Lower Bound	Upper Bound
L1	-.045403	.029255	13,805	0	-1.552	.121	-.102747	.011940

a. diff in slopes between  <38 and 51+

 

 

 

 

  /TEST = "diff in slopes between 38-50 and 51+" age3*newAge  0  1 -1.

The difference in slopes between 38-50 and 51+ is significant which means that the cut point at age 50 is justfiable.

 

Contrast Estimatesa,b

Contrast

Estimate

Std. Error

df

Test Value

t

Sig.

95% Confidence Interval

							Lower Bound	Upper Bound
L1	.121804	.024961	13,805	0	4.880	.000	.072877	.170731

a. diff in slopes between 38-50 and 51+

 

 

In the end, people under 38 and over 51 slow down by just around the same amount of time...but you slow down more in those middle ages.

 

Thanks again for taking the time to post. This was actually really fun to work though and see how it affected my results.

 

Carol