

Auxiliary DCAT  Probabilities Across... 

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Tracy posted on Thursday, September 03, 2015  6:42 pm



Hi I am unsure how to interpret the withinclass proportions (not the between class chisq tests). For example, in my reference Class 3 below, there is a .739 probability of the children being from a low decile school, versus high. But how do I tell if this difference is significant? Thanks. DECILE – Cat1=Low Cat2=High Prob S.E. Odds Ratio S.E. 2.5% C.I. 97.5% C.I. Class 1 Category 1 0.552 0.081 1.000 0.000 1.000 1.000 Category 2 0.448 0.081 2.303 1.909 0.454 11.694 Class 2 Category 1 0.794 0.095 1.000 0.000 1.000 1.000 Category 2 0.206 0.095 0.737 0.767 0.096 5.660 Class 3 Category 1 0.739 0.153 1.000 0.000 1.000 1.000 Category 2 0.261 0.153 1.000 0.000 1.000 1.000 ChiSquare PValue Degrees of Freedom Overall test 3.804 0.149 2 Class 1 vs. 2 3.231 0.072 1 trending Class 1 vs. 3 1.273 0.259 1 Class 2 vs. 3 0.082 0.775 1 


The test for the significance p1p2 is equivalent to the significance of p10.5, since p2=1p1. For that you can use the Ttest T=(0.7390.5)/0.153=1.562 with a twosided pvalue of 12%. 

Tracy posted on Thursday, September 03, 2015  8:43 pm



Hi Tihomir Thanks for your help, however Im still a bit confused. Sorry. Where is this method detailed? Also, isnt it (0.739.5)/.261 not .153 as .153 is class3/cat1 SE. Or am I just missing it completely? Lastly, how do you calculate the pvalue of 12% and does this 12% mean p=.88? 


Oh yeah it is .153, not .261  I just copied it wrong  the results are correct. This is the standard Ttest https://en.wikipedia.org/wiki/Tstatistic 

Tracy posted on Friday, September 04, 2015  12:38 am



Thanks. 

Tracy posted on Saturday, September 05, 2015  1:50 am



Okay, an extension to last question, what if you have a 3level distal category, reflecting low, mid, high: Class 3 Prob S.E. Category 1 0.886 0.108 Category 2 0.114 0.108 Category 3 0.000 0.000 For Class 3, can I use the following within class 3 probability ttest check? Ho: PrCat1 = PrCat2 = PrCat3 = .333, i.e. probability of distal occurrence equal across categories. H1: PrCat1 > .333, (with PrCat2/PrCat3 < .333). Am interested if low distal has higher probability of occurring in class 3 (which also reflects a low plateau group on DV test). tstatistic = (observed  expected)/SE = (.886  .333)/.108 = 5.12, signif. at p < .001 (one tailed test). Accept H1. Is this correct, and is this a valid use of the probability/SE in a ttest? 

Tracy posted on Monday, September 07, 2015  6:25 pm



Just wondered if anyone had an answer for my previous question? 


Yes  it looks correct. 

Tracy posted on Wednesday, September 09, 2015  6:20 pm



Thanks Tihomir. 

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