In a study examining the association between estrogen exposure and endometrial carcinoma outcome , the two by two table is shown below. Learning point: In a two by two table, for ease of calculation ensure that the outcome of interest is always at the top and the exposure on the left. According to the tablet above, individuals with endometrial cancer are 4.
Master Critical Analysis by completing the Psych Scene online critical analysis course. The Hub is a platform to share ideas, cases and concepts that bridge the gap between academia and the real world. Think about it as the real world textbook, a platform rich with experiences. If the odds ratio is greater than 1, then being a smoker is considered to be associated with having lung cancer since smoking raises the odds of having lung cancer.
The contingency table summarises the outcomes of each individual sampled in terms of whether Properties A and B are absent or present. It represents the joint frequency distribution of the two properties. The confidence level is the probability that the confidence interval contains the true odds ratio. The higher the confidence level the more certain you can be that the interval contains the true odds ratio. Formula This calculator uses the following formulae to calculate the odds ratio or and its confidence interval ci.
Discussion When the prevalence of the outcome is low, the odds ratio can be used to estimate the relative risk in a case-control study. Definitions Odds and odds ratio The odds of an event occurring is calculated as the ratio of the probability of a property being present compared to the probability of it being absent; this is simply the number of times that the property is absent divided by the number of times it is absent.
Contingency table The contingency table summarises the outcomes of each individual sampled in terms of whether Properties A and B are absent or present. Confidence level The confidence level is the probability that the confidence interval contains the true odds ratio. Tell us what you want to achieve Request Information. Sign up to our Newsletter. Please tick this box to confirm that you are happy for us to store and process the information supplied above for the purpose of managing your subscription to our newsletter.
This field is for validation purposes and should be left unchanged. The confidence interval indicates the level of uncertainty around the measure of effect precision of the effect estimate which in this case is expressed as an OR.
Confidence intervals are used because a study recruits only a small sample of the overall population so by having an upper and lower confidence limit we can infer that the true population effect lies between these two points. If the confidence interval crosses 1 e. This result was statistically significant. This result was not statistically significant. This result was equivocal.
A drug company-funded double blind randomised controlled trial evaluated the efficacy of an adenosine receptor antagonist Cangrelor vs Clopidogrel in patients undergoing urgent or elective Percutaneous Coronary Intervention PCI who were followed up for specific complications for 48 hrs as outlined in the diagram below Bhatt et al. This is a very basic introduction to interpreting odds ratios, confidence intervals and p values only and should help healthcare students begin to make sense of published research, which can initially be a daunting prospect.
However it should be stressed that any results are only valid if the study was well designed and conducted, which highlights the importance of critical appraisal as a key feature of evidence based medicine.
I do hope you enjoyed working through this and would appreciate any feedback on the content, design and presentational aspects of this tutorial. Bringing it all together — Real world example.
The correct answer is C. The 3 defects of the median. Forest plot at a glance. Why should students know about kappa value?
Key to statistical result interpretation: P-value in plain English. Surrogate endpoints: pitfalls of easier questions. How did they determine diagnostic thresholds: the stories of anemia and diabetes. Effect of platelet inhibition with cangrelor during PCI on ischemic events.
N Engl J Med. Apr 4; 14 Your email address will not be published. I think it is an excellent idea to explain what statistical analysis actually means in a simplified manner this may be a little too simplified.
Your explanation of odds ratio is just fine however it is important to look at how and if the researchers normalized the data. A confidence interval is actually a probabilistic statement about the repeatability of the trial as a whole with a different set of patients who meet the same criteria so saying that the confidence interval is 0.
As far as P Value goes you are correct that a P Value of. A P-Value of. Excellent info. That was extremely helpful. Please feel free to recommend the S4BE website as widely as possible and have a look at the many great articles and blogs in the site. A Phase III trial comparing anastrozole 1 and 10 milligrams , a potent and selective aromatase inhibitor, with megestrol acetate in postmenopausal women with advanced breast carcinoma. The total sample size is quite small at participants, which perhaps goes some way to explaining the wide confidence interval 0.
It is also a Ideally start by looking for a Cochrane review and you may well find this study included in a meta-analysis, which could help with a more definitive answer to whether anastrozole or megestrol acetate is more effective for the type of patients you are interested in or perhaps they might be just as effective, but one may have more tolerable side effect or safety better profile?
But would you be able to kindly explain what this hazard ratio means? So there are some additional statistical considerations, but that would perhaps be more detail than is needed for basic interpretation. The sentence in my paper was : The most active quartile of women had a similar risk of breast cancer as the least active odds ratio [OR], 1. Additional adjust ment for randomized treatment assignment, body mass index, alcohol consumption, menstrual and reproductive characteristics, use of oral contraceptives and postmenopausal hormones, and family history of breast cancer did not materially change the find ings OR, 1.
The way I interpreted this was : This means the odds of having breast cancer among physically active women is 1. Furthermore, 1. After adjusting for confounding factors, the odds ratio of physical activity is increased a little from 1. This result suggests that these variables were slightly strong confounders of the association between physical activity and breast cancer.
However, Adjusting for the confounding factors gives us an odds ratio further from unity 1 which means that the protection afforded by physical activity, if any, is stronger than the initial result led us to think. Tim, thanks for the great, concise tutorial. You have filled in the gaps where my texts left holes in my understanding.
Greatly appreciated. Could you possibly give me a bit more detail and send me a link to where it comes from so I can see it in context. Thanks for your example above. It helps a lot. The odds ratio is calculated by dividing the odds in the intervention arm by the odds in the control arm. So in your example of OR 0. Remember though the importance of confidence intervals and interpreting data in the context of the totality of evidence.
Hi Tim, I am studying for a health technology exam… need to learn the formula for calculating a confidence interval but there seems to be a number of different formulas around and am a little confused.
Any suggestions? Will you have to calculate the CI for an OR? Can you let me know what sort of thing you have an interest in and I will post a relevant worked example for you. Here is part of the study: Bleeding time associated with ticagrelor was slightly reduced following administration of desmopressin.
However, by 24 h post-ticagrelor dose, a difference between treatments was no longer evident. Inhibition of platelet aggregation by ticagrelor was not affected by co-administration of desmopressin Fig. Hi, thank you very much for your help. I have try to find in many pages how can I interpret A mul- tivariate analysis demonstrated that the independent risk factors for developing nosocomial B. In multivariate analysis, the independent risk factors for developing nosocomial B.
None of the patients with B. The effect of desmopressin on bleeding time and platelet aggregation in healthy volunteers administered ticagrelor. Thank you for this question, indeed the interpretation of the confidence interval is different with this study because the outcome was a continuous variable, which was time.
So as outlined in the tutorial above an odds ratio is a relative measure of effect so the line of no difference is one. However a mean is an absolute measure of effect so the line of no difference is 0. For example if a placebo controlled randomised controlled trial evaluated the effect of a new medication to reduce blood pressure, the results might show:.
That is there was no difference, hence the line of no difference in absolute measures of effect is 0. Therefore although GLS mean AUEC was lower in the ticagrelor plus desmopressin arm compared to ticagrelor alone arm the confidence interval is from a minus number My suggestion on this is to first consider that the NNT is calculated as the reciprocal of the absolute risk reduction ARR.
Its worth noting at this point that ARR is an absolute measure so the line of no difference is 0 There is an explanation of this in a slightly different context in the reply i posted for Kates question.
Or if one is negative and one is positive it indicates that there is not a significant difference. However the NNT confidence interval can produce some intriguing numbers, which at first seem quite baffling. This essentially arises from the fact that if:. The next point to consider is that if the NNT indicates the number needed treat for a desirable outcome, for instance if a randomised placebo controlled trial of an antiplatelet therapy for the prevention of stroke with an outcome of mortality had an ARR 0.
However the reciprocal of This is of course the number needed to harm NNH. This means that an ARR of 0. So a more precise way of writing this taking into account 0 would be. Crude Odds Ratio — the odds ratio calculated using just the odds of an outcome in the intervention arm divided by the odds of an outcome in the control arm.
Adjusted Odds Ratio — is the crude odds ratio produced by a regression model which has been modified adjusted to take into account other data in the model that could be for instance a prognostic baseline variable. Logistic regression — a type of statistical model used to evaluate a binary outcome e.
Is Odds Ratio always with in the CI in between the interval? Yes always, the OR is the estimate of an intervention effect in a study population, which is of course only a representative sample of the whole population.
The confidence interval is always either side of this because it represents the uncertainty around the intervention effect. Hi Tim Thank you so much for your informative posts- you really explain this crazy terminology in a way that we can all understand. Do you have any experience in working with Epi Info?
Thanks, 3rd year mature student nurse doing my dissertation and this has really helped explain things to me. Just one question though, when you say that the CI is 0.
If the CI is 0. Thanks Tim! Did you know this blog has been viewed over 83, times since it was published!!!! If logistic regression analysis shown OR Does this OR correct? Does OR results can be range from zero to infinity? Thank you. However is certainly an unusual result. Once the model produces a result the log transformed OR antilog is calculated to convert it back into a ratio for interpretation. Hi Tim, Thank you very much for post. I has been helpful to me. However, I am just new in using logistic regression, and doing my research in finance.
My research is on effects of financial behaviour on the decision to use financial services. Therefore, the event is use or non use. I have run logistic regressions, some variables have OR more than 1.
For instance saving behaviour has an OR of 4. Would it make sense? Hi Tim, great article. I do have a question: In a paper I am appraising it states that patients in the intervention arm had a greater likelihood of response at 24 hours vs the active control.
It reports an Odds Ration of 2. So are they saying one thing, and letting their OR report that control was actually better? Or am I misunderstanding? Any help would be great, Thanks. I have a brief question please. Can I calculate the odds ration between the main group given a drug and the comparison group if these groups have different total number? For instance, in a RCT, we have group A which is given bromide compared to group B which is given phenobarbital.
In group A, a total number of 20 people were included and 10 of them developed side-effects. In group B a total number of 30 people were included and 5 of them developed side effects. Your explanation was quite simple and understandable. Hi Tim: I have to organize a table to show data with odds ratio, confidence intervals: Which of these data goes fiest on the table: the P or Odds ratio?
If OR is 1. Then the OR for inserting a needle causes pain is 0. How is this interpreted? I like the simplicity of your presentation with the 3 quick check and then bringinging all together. Hi Thanks so much for these explanation. But how does this relate to standardized coefficient Gotten when one does a binary regression on spss or other statistical package. Pls interprete this result.
I was just working with Spss a while ahead of ma search to this very potent solution for odds ratio abd confidence interval. I thank you anyways.
I am also Physiologist would you please suggest me from experience the area of physiology i shall continue for Phd program? I love this anyways…. Thank you so much for the tutorial and explanations, Tim! They were so helpful for my EBM project.
Dear Tim, I assure you that, you can be a wonderful professor. You can clearly present in a simple manner for a good learner to understand. Keep sharing. Hi i have a question it is related to statically significant for my assignment. I have an odds ration of 2.
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