# Probability calculator

## Multiple Event Probability Calculator

We are often interested in programmes that will reduce rates of smoking in the community. In particular, we often see studies that focus on reducing the rates of smoking in pregnant women, with the aim of achieving health improvements for both women and their newborn babies. Let's consider a study designed to test a smoking cessation programme for pregnant women. From table 1 , we see that women were randomised to either the intervention group where pregnant women received the smoking cessation programme , or a control group pregnant women did not receive the smoking cessation programme.

On first look, it appears that the smoking cessation intervention seems to have been successful. How certain can we be that this is a relationship we can be confident about? The next step in exploring the veracity of this apparent relationship between the smoking cessation programme and the number of pregnant women who quit smoking, might be to use a method of statistical analysis involving ORs.

Try this yourself to verify that you can obtain this result, or enter the table 1 data into an online OR calculator. So, from our calculated OR of 2. However, as with virtually all inferential statistics analyses, we must investigate further to determine how likely it is that this result could have occurred simply due to the characteristics of the particular sample selected for the study. This calculation can also be easily obtained using an online CI calculator.

The lower bound implies that women from the intervention group would be approximately 1. The upper bound of 4. Note that an OR of exactly one implies that there is no difference in outcome between the intervention and control groups. The reported p value of 0. OR analyses can also be used in efforts to control for potential confounding factors in research, as we will see in the next example. This time the example contains data from a published study designed to examine the potential benefits of using corticosteroids in late preterm pregnancy.

In the treatment group, only 14 of the 83 infants were admitted to the NICU Note that the CI implies that, although the true odds of babies in the treatment group requiring NICU admission may be as little as one quarter those of the control group, the interval does include the value of one.

The site has links to other online material about the first world war, including a medical bibliography of the war raven. The full text of the evidence based medicine journal Bandolier is available free on www. For more comprehensive information, visit Netting the Evidence www. The only snag is that, for most of them, you must have a subscription. In the near future the Highwire Press site will allow you to search all its journals in one go, and will also feature a Medline service.

A good starting place for exploring it all is the Alt. There is also an exhaustive set of links to other ER pages and sites. British viewers can discuss the show on the newsgroup uk. The first issue is available in full on the web at www. I am on its editorial board. Abstracts are available for recent theses. For full appreciation of the site, however, you must have the Shockwave plug-in available from www.

Compiled by Mark Pallen email ku. National Center for Biotechnology Information , U. Journal List BMJ v. Huw Talfryn Oakley Davies , lecturer in health care management , a Iain Kinloch Crombie , reader in epidemiology , b and Manouche Tavakoli , lecturer in health and industrial economics a.

Accepted Feb See " So stroke units save lives: This article has been cited by other articles in PMC. Summary points If the odds ratio is interpreted as a relative risk it will always overstate any effect size: The extent of overstatement increases as both the initial risk increases and the odds ratio departs from unity.

However, serious divergence between the odds ratio and the relative risk occurs only with large effects on groups at high initial risk. Therefore qualitative judgments based on interpreting odds ratios as though they were relative risks are unlikely to be seriously in error. In studies which show reductions in risk odds ratios of less than one , the odds ratio will never underestimate the relative risk by a greater percentage than the level of initial risk. Odds and risk There is a problem with odds: Relative risks and odds ratios The relative risk of one group compared with another is simply the ratio of the risks in the two groups.

Odds ratio as an approximation of relative risk When faced with an odds ratio, we want to know the discrepancy between that odds ratio and the relative risk. Open in a separate window. Does the discrepancy influence our interpretation? Conclusion The difference between the odds ratio and the relative risk depends on the risks or odds in both groups.

Netlines Lest we forget However, you can calculate an odds ratio and interpret it as an approximation of the risk ratio, particularly when the disease is uncommon in the population. Calculate the odds ratio for the tuberculosis data in Table 3. Would you say that your odds ratio is an accurate approximation of the risk ratio? The more common the disease, the further the odds ratio is from the risk ratio. Skip directly to search Skip directly to A to Z list Skip directly to navigation Skip directly to page options Skip directly to site content.

Examples of Surveillance Appendix C. Examples of Surveillance Appendix D. Frequency Measures Section 2: Morbidity Frequency Measures Section 3: Mortality Frequency Measures Section 4: Natality Birth Measures Section 5: Measures of Association Section 6: Recommend on Facebook Tweet Share Compartir.

Measures of Association The key to epidemiologic analysis is comparison. Risk ratio Definition of risk ratio A risk ratio RR , also called relative risk, compares the risk of a health event disease, injury, risk factor, or death among one group with the risk among another group.

Method for Calculating risk ratio The formula for risk ratio RR is: Risk of disease incidence proportion, attack rate in group of primary interest Risk of disease incidence proportion, attack rate in comparison group.

Calculating Risk Ratios Example A: Rate for group of primary interest Rate for comparison group. Calculating Rate Ratios Continued Public health officials were called to investigate a perceived increase in visits to ships' infirmaries for acute respiratory illness ARI by passengers of cruise ships in Alaska in Using the data in Table 3.

Rate ratio comparing current smokers with nonsmokers Rate ratio comparing ex-smokers who quit at least 20 years ago with nonsmokers What are the public health implications of these findings? Calculating Odds Ratios Use the data in Table 3. In a case-control study, investigators enroll a group of case-patients distributed in cells a and c of the two-by-two table , and a group of non-cases or controls distributed in cells b and d.

Measures of Public Health Impact.

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