topic Re: Absurd value for odds ratio in logisitc regression output. in Statistical Procedures

Absurd value for odds ratio in logisitc regression output.

Ashwini_uci — Wed, 20 Feb 2013 20:55:49 GMT

Hi there,

I have run a multivariate logistic regression model and one of the odds ratio values I have got it is

<0.001 (<0.001- >999.999)

I am not sure what this means and why I have got such a value. Does this mean there is something wrong with my model. Is this a sample size issue? The original sample size is 914 and after weighting it comes to be 4500

The model is as below. Greatly appreciate any help or advice.

With such absurd value, can I still use the results?

proc logistic data =library.nismipostcath2 descending ;

class died race1(ref=first) ZIPINC_QRTL(ref=first) dm_all (ref=first) cm_htn_c (ref=first) morbidobesity (ref=first) hyperlipidemia (ref=first) cm_perivasc (ref=first) chf (ref=first) afib (ref=first) liverdisease(ref=first) ckd(ref=first)/param=ref;

model died= age race1 ZIPINC_QRTL dm_all cm_htn_c morbidobesity hyperlipidemia cm_perivasc chf afib liverdisease ckd ;

where pci_postcath=1 and postcathcompli1=1 and female=1;

weight discwt;

title 'Logi Reg in-hosp mortality in POST-PCI- MI women with post cath complications ';

run;

quit;

Some of the odds ratio values are as below.

Diabetes Mellitus	1.36 (1.05 - 1.77)	0.02
Hypertension	1.09 (0.83 - 1.42)	0.5364
Morbid Obesity	<0.001 (<0.001- >999.9)	0.9764
Hyperlipidemia	0.43 (0.33 - 0.55)	<0.0001
Perivascular disorders	1.81 (1.41- 2.34)	<0.0001
Chronic Heart Failure	4.11 (1.43 - 11.85)	0.0088
Atrial Fibrillation	1.61 (1.21 - 2.15)	0.001
Liver Disease	1.57 (0.63 - 3.94)	0.3335
Chronic Kidney Disease	0.98 (0.72 - 1.35)	0.9226

Thanks much!

Ashwini

Re: Absurd value for odds ratio in logisitc regression output.

Reeza — Wed, 20 Feb 2013 20:59:12 GMT

From what I saw yesterday it seems like if you're using weights for sampling then you need to use proc surveylogistic not proc logistic.

Additionally you do have estimates of negative infinity to infinity when you have a very small numbers in that particular variable (check your table of characteristics or table 1 or run a proc freq to verify).

Usually if it's another reason SAS will issue a warning in the log or the output.

Re: Absurd value for odds ratio in logisitc regression output.

1zmm — Sat, 23 Feb 2013 15:24:26 GMT

The odds ratio of zero for the variable, MORBIDOBESITY, implies that none of the study subjects meeting the WHERE condition of

PCI_POSTCATH=1 and POSTCATHCOMPLI1=1 and FEMALE=1.

died. So, this is a sample size issue.

You can include the MODEL statement option, FIRTH, following a forward slash ("/") after the independent variables to see one approach for dealing with this issue. Another approach is to increase your sample size. A third approach is to make your WHERE condition less restrictive.

I agree with Reeza that you should use PROC SURVEYLOGISTIC instead of PROC LOGISTIC when your sample respondents have weights.

Re: Absurd value for odds ratio in logisitc regression output.

Ashwini_uci — Mon, 25 Feb 2013 18:40:17 GMT

Hi there

Thanks for our response!

It seems that I cannot increase the sample size or change the restrive criteria.

What I wonder is if it is still alright to use these results, that I posted in the original post. Morbid obesity is one of the independent variables in the model, my interest variable is diabetese and hypertension. I hope there is no harm using these results.

Appreciate your response.

Re: Absurd value for odds ratio in logisitc regression output.

Ashwini_uci — Mon, 25 Feb 2013 18:40:46 GMT

Hi there

Thanks for our response!

It seems that I cannot increase the sample size or change the restrive criteria.

Appreciate your response.

Re: Absurd value for odds ratio in logisitc regression output.

Reeza — Mon, 25 Feb 2013 18:52:46 GMT

Take out morbid obesity and see if the results change.

Re: Absurd value for odds ratio in logisitc regression output.

SteveDenham — Mon, 25 Feb 2013 18:55:13 GMT

I think you are missing a critical point. When there are sampling weights attached to the design, PROC LOGISTIC does not give the correct results. You MUST switch to PROC SURVEYLOGISTIC to get meaningful results. See many of @Reeza's responses to your question.

Steve Denham

Re: Absurd value for odds ratio in logisitc regression output.

Ashwini_uci — Mon, 25 Feb 2013 22:47:41 GMT

Thanks for your response Reeza and for suggesting me using Proc surveylogistic!. I did switch to proc surveylogistic; good thing- my results haven't changed at all.

**I am curious to know why surveylogistic is more meaningful than logistic where we are able to add the weight statement and get the SAME results as suyveylogistic? What and how does proc surveylogistic do differentlythat logisitc while analysing the weights?

** I read on the sasupport that PROC SURVEYLOGISTIC incorporates complex survey sample designs, including designs with stratification, clustering, and unequal weighting. Would you please explain what unequal weighting refers to?

** So does it finally mean that whenever we are analysing any unweighted database, proc surveylogisitc is a better choice over proc logistic?

Appreciate your response

Thanks

Ashwini

Re: Absurd value for odds ratio in logisitc regression output.

Reeza — Tue, 26 Feb 2013 00:10:24 GMT

You shouldn't get the same results. Check the standard error and the confidence intervals of your odds ratio, though the parameter estimates are probably fine.

The SAS Docs specify, in the weight statement for proc logistic, that it does not calculate the variance properly, to use surveylogistic instead.

I don't know what unequal weighting means.