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Logistic regression for ordered dependant variable with more than 2 levels
1.
Multinomial Logistic Regression
Models January 1, 2013 ©Arup Guha - Indian Institute of Foreign Trade - New Delhi, India
2.
Logistic regression
CAN handle dependant variables with more than two categories It is important to note whether the response variable is ordinal (consisting of ordered categories like young, middle-aged, old) or nominal (dependant is unordered like red, blue, black) Some multinomial logistic models are appropriate only for ordered response It is not mathematically necessary to consider the natural ordering when modeling ordinal response but, Considering the natural ordering Leads to a more parsimonious model Increase power to detect relationships with other variables January 1, 2013 ©Arup Guha - Indian Institute of Foreign Trade - New Delhi, India
3.
Applying logistic
regression considering the natural order is done using a modeling technique called the “Proportional Odds Model” Say the dependant variable Y has 4 states measuring the impact of radiation on the human body; fine, sick, serious,dead Let p1=prob of fine, p2=prob of sick, p3=prob of serious, p4=prob of dead Let us define a baseline category: fine, since this is the normal stage (we shall see why we need this later) January 1, 2013 ©Arup Guha - Indian Institute of Foreign Trade - New Delhi, India
4.
What if we break up the modeling of the 4 level ordered dependant into 3 binary logistic situations: 1 – (fine,sick), 2 – (fine,serious),3 – (fine,dead)? Then we would have 3 logit equations: Log(p2/p1)=B11+B12X1+B13X2 Log(p3/p1)=B21+B22X1+B23X2 Log(p4/p1)=B31+B32X1+B33X2 X is the degree of radiation dummy with 3 levels so broken into 2 binary dummies So, 9 parameters to be estmated January 1, 2013 ©Arup Guha - Indian Institute of Foreign Trade - New Delhi, India
5.
Now consider an alternative model for the same situation Cumulative logit model: L1=log(p1/p2+p3+p4) L2=log(p1+p2/p3+p4) L3=log(p1+p2+p3/p4) The obvious way to introduce covariates is L1=B11+B12X1+B13X2 L2=B21+B22X1+B23X2 L3=B31+B32X1+B33X2 January 1, 2013 ©Arup Guha - Indian Institute of Foreign Trade - New Delhi, India
6.
Let us simplyfy the model by specifying that the slope parameters are identical over the logit equations. Then, L1=A1+B1X1+B2X2 L2=A2+B1X1+B2X2 L3=A3+B1X1+B2X2 This is the proportional odds cumulative logit model January 1, 2013 ©Arup Guha - Indian Institute of Foreign Trade - New Delhi, India
7.
Suppose that
the categorical outcome is actually a categorized version of an unobservable (latent) continuous variable which has a logistic distribution The continuous scale is divided into five regions by four cut-points c1, c2, c3, c4 which are determined by nature If Z ≤ c1 we observe Y = 1; if c1 < Z ≤ c2 we observe Y = 2; and so on Suppose that the Z is related to the X’s through a linear regression Then, the coarsened categorical variable would be related Y will be related to the X’s by a proportional- odds cumulative logit model January 1, 2013 ©Arup Guha - Indian Institute of Foreign Trade - New Delhi, India
8.
Let us go back to the model L1=A1+B1X1+B2X2 L2=A2+B1X1+B2X2 L3=A3+B1X1+B2X2 Note that Lj is the log-odds of falling into or below category j versus falling above it Aj is the log-odds of falling into or below category j when X1 = X2 = 0 B1 is the increase in log-odds of falling into or below any category associated with a one-unit increase in Xk, holding all the other X-variables constant. Therefore, a positive slope indicates a tendency for the response level to decrease as the variable decreases January 1, 2013 ©Arup Guha - Indian Institute of Foreign Trade - New Delhi, India
9.
Our example of 4 levels of impact of radiation corresponding to 3 levels of radiation proc logistic data=radiation_impact; freq count; class radiation / order=data param=ref ref=first; model sickness (order=data descending) = radiation / link=logit aggregate=(radiation) scale=none; run; January 1, 2013 ©Arup Guha - Indian Institute of Foreign Trade - New Delhi, India
10.
Freq=count This is important for specifying grouped data Count is the variable that contains the frequency of occurrance of each observation In its absence, each row would be considered a separate row of data Class=radiation Specifies that radiation is a classification variable to be used in the analysis SAS would automatically generate n-1 binary dummies for n categories of radiation with param=ref option January 1, 2013 ©Arup Guha - Indian Institute of Foreign Trade - New Delhi, India
11.
Order=data Simply tells SAS to arrange the response categories in the order they occur in the input data 1,2,3,4 Param=ref This implies that there is going to be dummy coding for the classification variable ‘radiation’listed in class Ref=first Designates the first ordered level, in this case ‘fine’ as the reference level January 1, 2013 ©Arup Guha - Indian Institute of Foreign Trade - New Delhi, India
12.
Order=data descending This tells SAS to reverse the order of the logits So, instead of the cumulative logit model being L1=log(p1/p2+p3+p4) L2=log(p1+p2/p3+p4) L3=log(p1+p2+p3/p4), it becomes L1=log(p4/p1+p2+p3) L2=log(p4+p3/p1+p2) L3=log(p4+p3+p2/p1) Now, a positive B1 indicates that a higher value of X1 leads to greater chance of radiation sickness January 1, 2013 ©Arup Guha - Indian Institute of Foreign Trade - New Delhi, India
13.
Link=logit fits the cumulative logit model when there are more than two response categories Aggregate=radiation Indicates that the goodness of fit statistics are to be calculated on the subpopulations of the variable: radiation Scale=none No correction is need for the dispersion parameter To understand this, read up. This happens when the goodness of fit statistic exceeds its degrees of freedom and need to be corrected for January 1, 2013 ©Arup Guha - Indian Institute of Foreign Trade - New Delhi, India
14.
When we fit this model, the first output we see: Score Test for the Proportional Odds Assumption Chi-Square DF Pr > ChiSq 17.2866 21 0.6936 Null hypothesis is that the current proportional-odds cumulative logit model is true Seems like we fail to reject the null and so can proceed to the rest of the output under the current assumption January 1, 2013 ©Arup Guha - Indian Institute of Foreign Trade - New Delhi, India
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Ultimately we are interested in the predicted probabilities OUTPUT <OUT=SAS-data-set><options> Predicted= For a cumulative model, it is the predicted cumulative probability (that is, the probability that the response variable is less than or equal to the value of _LEVEL_); PREDPROBS=I or C Individual|I requests the predicted probability of each response level. CUMULATIVE | C requests the cumulative predicted probability of each response level January 1, 2013 ©Arup Guha - Indian Institute of Foreign Trade - New Delhi, India
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