## Sunday, February 12, 2012

### How to interpret odds ratios that are smaller than 1?

Here is a question posted on the web about the interpretation of odds ratios that are less than 1.
"I know that OR estimates= 1 mean that both groups/categories have the same odds. I also know that if OR estimates are greater than 1, e.g, 1.24 for Young vs. Old persons, then I can say: Young people have 24% increase in the odds of living in an apartment than older people. Or, I also know I can say, for example, for an OR of 0.322 Non-White vs. White, that the odds of Whites are 1/.322 = about 3 times higher than those of Non-Whites, to live in a house they own. Now, how would I say the odds are of a NON-White person in the example above, to live in a house they own? Is is 1-.322=.678 less likely, with respect to odds, to live in a house they own? Or, similarly, they have 67.8% lower odds to live in a house they own? "
If we have to say the odds for a Non-White person, we may say "Non Whites have odds .322 times as great as those of Whites".

"the odds of an event is the number of those who experience the event divided by the number of those who do not. It is expressed as a number from zero (event will never happen) to infinity (event is certain to happen). Odds are fairly easy to visualise when they are greater than one, but are less easily grasped when the value is less than one. Thus odds of six (that is, six to one) mean that six people will experience the event for every one that does not (a risk of six out of seven or 86%). An odds of 0.2 however seems less intuitive: 0.2 people will experience the event for every one that does not. This translates to one event for every five non-events (a risk of one in six or 17%). "
Another webblog described the issue in interpreting the odds ratio that is less than one.
"When you are interpreting an odds ratio (or any ratio for that matter), it is often helpful to look at how much it deviates from 1. So, for example, an odds ratio of 0.75 means that in one group the outcome is 25% less likely. An odds ratio of 1.33 means that in one group the outcome is 33% more likely."
In an article "The odds ratio: calculation, usage, and interpretation" in Biochemia Medica, the author clear suggest converting the odds ratio to be greater than 1 by arranging the higher odds of the evnet to avoid the difficulties in interpreting the odds ratio that is less than 1.
“An OR of less than 1 means that the first group was less likely to experience the event. However, an OR value below 1.00 is not directly interpretable. The degree to which the first group is less likely to experience the event is not the OR result. It is important to put the group expected to have higher odds of the event in the first column. It is not valid to try to determine how much less the first group’s odds of the event was than the second group’s. When the odds of the first group experiencing the event is less than the odds of the second group, one must reverse the two columns so that the second group becomes the first and the first group becomes the second. Then it will be possible to interpret the difference because that reversal will calculate how many more times the second group experienced the event than the first. If we reverse the columns in the example above, the odds ratio is: (5/22)/(45/28) = (0.2273/1.607) = 0.14 and as can be seen, that does not tell us that the new drug group died 0.14 times less than the standard treatment group. In fact, this arrangement produces a result that can only be interpreted as “the odds of the first group experiencing the event is less than the odds of the second group experiencing the event”. The degree to which the first group’s odds are lower than that of the second group is not known.”
In practice, when dealing with the odds ratio less than 1, when possible, I almost always try to reverse the column or recode the response variable to get the odds ratio larger than 1 before I do an interpretation. It is easier for people (especially non-statisticians) to understand the odds ratio with the value greater than 1.

In an example below, the treatment group is actually less effective in terms of the response.

 Treatment Failure (0) Success (1) No (0) 21 30 Yes (1) 32 17

The following SAS code can be easily used to calculate the odds ratio:
Data test; input Trt resp count; datalines;
1 1 17
1 0 32
0 1 30
0 0 21
;
proc logistic data=test descending; weight count; model resp=trt;
run;

From the SAS outputs, we get the odds ratio of 0.372, which indicates that the treatment group has odds 0.372 times lower compared to the non-treatmetn group in terms of the success. The interpretation is somewhat difficult to understand.

The program can be easily revised to calculate the odds ratio of failure rate, which gives an odds ratio of 1/0.372 = 2.689. The odds ratio can be intepretated as "the odds of achieve the success in non-treatment group is 2.689 times higher than that in treatment group".

proc logistic data=test; weight count; model resp=trt;
run;

In SAS PROC Logistic, with descending option, probability modeled is response=1 (success); without descending option, probability modeled is response=0 (failure);

Dlkc said...

HI,

I really appreciated this question! I am currently trying to write an odds ratio sentence for a continuous variable that is less than one. How do I go about writing this when there are not categories?

Many thanks!

Danielle

Web blog from Dr. Deng said...

You are dealing with a continuous predictor variable. The interpretation for odds ratio is straight forward.

If you use a unit of 1 for the continuous variable, you would just say that the odds for xxx is xx% higher per unit of xxx.

if you use a unit of 10 for the continuous predictor variable, you would just say that the odds for xxx is xx% higher per 10 unit of xxx.

If you use SAS software for your calculation, Proc Logistic has an option UNITS to allow you to specify it is 1 unit or 10 unit or any other unit you specify.

http://support.sas.com/documentation/cdl/en/statug/63962/HTML/default/viewer.htm#statug_logistic_sect026.htm

If the odds ratio for 1 unit change in a continuous predictor variable is small, it is preferable to use a larger unit (such as 5 or 10).

Anonymous said...

how should we interpret an OR with value 0?

Web blog from Dr. Deng said...

Odds ratio is around 1. You can not have a odds ratio = 0.

There may be a situation with odds ratio of A/B close to 0 (very very small). In this case, if you flip round, it means the odds ratio of B/A is very large.

Ms. Gina said...

Thank you! This was so helpful and clear.

Anonymous said...

Many thanks for the above clarification. how do you report an OR of 1.24 when comparing two groups for a clinical outcome?

Anonymous said...

you would say "the odds for group A are 24% higher than the odds for group B for a clinical outcome".

for details, you may refer to http://www.ats.ucla.edu/stat/mult_pkg/faq/general/odds_ratio.htm

Anonymous said...

I have these results:
OR (CI) P- value
1.3(0.3-5.4) 0.70

2.5(1.2-5.3) 0.01

1.2(0.04-0.7) 0.01

1.1(0.6-2.3) 0.67

3.4(2.0-5.7) 0.01
How would I interpret them?

Web blog from Dr. Deng said...

For the first one and the fourth one, the odds ratio is not significant (p-value >0.05 and the 95% confidence interval include 1. you don't have to interpret the odds and just simply conclude that there is no difference between two groups.

For the second one, you may say "the odds of achieving the success in one group is 2.5 times higher than that in control group" or something like that.

for the third one, you may have an error because the confidence interval does not cover the point estimate of odds ratio.

For the first one, you may say "the odds or achieving the success in one group is 3.4 times higher than that in control group" or something like that.

Anonymous said...

How would I interpret the following odds for a survey item response. Thanks in advance.
Strongly Agree (reference group)
Disagree: 0.24 (0.12-0.47), p<0.001
Agree: 0.27 (0.16-0.47), p<0.001
Strongly Agree: 0.17 (0.10-0.31), p<0.001

Anonymous said...

Odds ratio? Can it be negative? Or it is always positive?

Web blog from Dr. Deng said...

Odds ration can not be negative. It is always positive. Whether or not there is a protective effect or negative effect will be judged by the odds ratio greater than 1 or less than one.

The same applies to other ratios such as hazard ratio, risk ratio

Anonymous said...

There seems to be an issue with the results you presented below. You have 'Strongly Agree' category presented twice. Not sure if you are talking about the odds ratio or odds. Odds ratio is supposed to be the comparison of two groups or two categories.

How would I interpret the following odds for a survey item response. Thanks in advance.
Strongly Agree (reference group)
Disagree: 0.24 (0.12-0.47), p<0.001
Agree: 0.27 (0.16-0.47), p<0.001
Strongly Agree: 0.17 (0.10-0.31), p<0.001

Anonymous said...

Sorry about the typo as I was typing from my mobile device. The reference group is Strongly Disagree.
Strongly Disagree (reference group)
Disagree: 0.24 (0.12-0.47), p<0.001
Agree: 0.27 (0.16-0.47), p<0.001
Strongly Agree: 0.17 (0.10-0.31), p<0.001

The output is from the multivariate binary logistic regression showing odds ratio, 95% CI and p value.

BTW, the Strongly Disagree, Disagree, Agree, and Strongly Agree responses were each dummy coded as 0 and 1 (and then compared to the regular variable with the original 4 Likert categorical responses and the output were the same).

What makes the output hard for me to interpret is that the odd ratios are very similar regardless of the level of agreement. Thanks again for sharing your thoughts on this.

Sirwan Hoshi said...

Thank you, dr, for this post.

This probably is a silly question, but I need to ask it and I hope you have the time to answer it:

The transformation of an OR lower than 1 by using 1/OR - can this be used on the CI as well? I shouldn't be a problem since it's "Only" a transformation that does not change anything else - right?

Web blog from Dr. Deng said...

yes, if you transform the point estimate of the OR, the corresponding confidence interval will also be transformed.

Anonymous said...

Hi,

How can interpret the odds of a continuous variable (P sig .....odds=29.4)
and for (P sig.....odds 0.98)
Thanks

Web blog from Dr. Deng said...

interpreting the odds ratio for continuous variable depends on the unit of the continuous variable. It will mean the odds increase for every one unit increase in the continuous variable measure. You can also computer the odds ratio for every 10 units or any number of increase in the continuous variable measure.

Anonymous said...

Dear Dr Deng,

Concerning of our explanatory variable, the behaviour of participants in dummy variable in bivariate analysis using Chi-Square test. We noted participants which given coded 1, who did not use mosquito nets 3.12% having malaria than participants which given coded 0 [reference] who used mosquito nets at night as much 3.84% having malaria. OR = 0.805; Pv < 0.0000. [Binary logistic regression].In addition, participants were not using insecticide-treated nets (ITNs) which coded 1, amongst 3.08% having malaria than participants which given coded 0 [reference] who use insecticide-treated nets (ITNs) as much 5.89% contracting malaria. OR = 0.508; Pv < 0.0000. [Binary logistic regression]. as if, from this the percentage each group of participants is not relevant to basic theory. Please advice.

Regarding interpret odds ratios that are smaller than 1, is my interpretation below true?

We observed that variables mosquito nets have OR = 0.805 means that in one group the outcome is 19.5 less likely malaria. Variables Insecticide-treated bed nets (ITNs) have OR = 0.508 means that the first group (participants who use ITNs) was less likely to experience malaria 49.2 than who did not use ITNs with P < 0·001. The theory result of odd ratio is connected with basic theory

Sincerely,

Unknown said...

Hello-
I understand you recommend flipping the groups so you have a positive odds ratio, instead of the negative effect of an odds ratio less than 1. However, how would you actually interpret and state something for odds ratio of .15? Would you say this group is 85% less likely to have the event occur?

Thanks,

Web blog from Dr. Deng said...

yes, this is the way the people often use. It is more accurate to say that the odds of having an event is 85% less than the odds of not having an event or something along this line.

Unknown said...

I disagree. Holcomb et al. (2001) report that 26% of authors in top-tier medical journals explicitly misinterpreted odds ratios as relative risk ratios, which are ratios of probabilities rather than odds. So, even seasoned researchers have tendency to use probabilistic language when discussing odds ratios. For example, often people say "boys are 4.91 times more likely to be recommended to remedial reading than girls" or "boys are 4.91 times as likely to b recommended..." Technically, the odds ratio should be summarized as "the odds of boys being recommended are 4.91 times the odds of girls being recommended". Well, odds ratios tend to exaggerate effects as Davies et al. (1998) demonstrate which is particularly egregious when the outcome being examined is not rare. Whenever possible, I would recommend using and calculating conditional probabilities.

Having said that, the method of saying "85% less" and so on is not accurate. You should know that odds ratios are asymmetrical. For example, the entire infinity of decreasing relationships must fit between 0.000001 and 0.99999, while the same infinity of positive relationships fits in a much larger space between 1.00001 and well, infinite number.

Okay so for readers wanting to hear a simple answer, here it is: My advice is to use "as likely" rather than "less or more likely." Another way of staying clear of trouble is to use language like "the odds of GROUP X having an outcome is that of GROUP Y."

Joseph said...

Dear Dr Deng

How are you? I have this paper on Physical activity for smoking cessation in pregnancy: randomized control trial. https://www.bmj.com/content/350/bmj.h2145

This is the question i have and i cannot understand or answer it

1. Critically comment about the depth of precision of result

Below were the results

Outcomes Grp1(n=392) Grp2(n=393) Odds ratio(95% CI) adjusted odds ratio† (95% CI)
Primary:
At end of pregnancy 30 (8) 25 (6) 1.21 (0.70 to 2.10) 1.37 (0.78 to 2.41)

Secondary:
4 weeks post-quit day, 50 (13) 61 (16) 0.79 (0.53 to 1.18) 0.87 (0.57 to 1.31)
6 months after birth 24 (6) 16 (4) 1.55 (0.81 to 2.97) 1.66 (0.82 to 3.37)

Thanks A Million

Unknown said...

If the odds for a categorical variable, yes or no where yes is used as a reference, produce an OR of 0.83..how do we interpret? Thank you in advance.

Unknown said...

Pt who are suufering for dm has Odds ratio is. 34, reference pt who suffer for less than 5 y. What ll be interpretations?

annieblogs_xo said...

Stuck on this study question, need help.

In a recent article in the Annals of Improbable Research, Allen reported that the patients suffering from the Psoriasis who used his proprietary oil elixir properly resulted in an odds-ratio = .6 relative to controls who did not use his elixir, whose probability of suffering from the Psoriasis was only p = .3.

(a) Describe the meaning of the odds-ratio for elixir users in words.
(b) What was the probability of suffering from the “Heartbreak of Psoriasis” for the elixir
group?

0.00001% odds meaning

Busola said...

Thank you so much for this, it was very helpful. Please can this approach be used for values reported from regression analysis as well?

Unknown said...

Unknown said...

I really appreciate the way you made clear. My question is that; I am analysis measles vaccination uptake and associated factors, for example, one of my variable is "residence" (rural vs urban). I made the urban as the reference and I got an AOR of less than one. How can I interpretate.

Unknown said...

I have a question. I used anthropometric parameters such as mid arm circumference to predict low birth weight. So my outcome variable was coded as low birth weight 1 and normal weight 0. My OR for mid arm circumference is 0.09. How do I interpret this? Can I say with each unit increase in mid arm circumference the odds of predicting LBW is 0.09 or

say 1/0.09= 11 so the odds of predicting LBW is 11 times less likely with a unit increase in mid arm circumference. Pls help

Unknown said...

I have a question. I used anthropometric parameters such as mid arm circumference to predict low birth weight. So my outcome variable was coded as low birth weight 1 and normal weight 0. My OR for mid arm circumference is 0.09. How do I interpret this? Can I say with each unit increase in mid arm circumference the odds of predicting LBW is 0.09 or

say 1/0.09= 11 so the odds of predicting LBW is 11 times less likely with a unit increase in mid arm circumference. Pls help

Unknown said...

Pls pls pls

Anonymous said...

hi, can a model have all parameters that have p value less than 0.05 but the odd ratio is less than 1? and it is not continuous variable. how should i interpret them?

Web blog from Dr. Deng said...

check the order of the outcome variable or the explanatory variables. If you use SAS proc logistic, you may use the /descending option.