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26 Cards in this Set
- Front
- Back
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What is prevalence?
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number of instances of a disease in a population at a designated time
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How is point prevalence different than period prevalence?
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Point prevalence is the occurrences at a given instant, while period prevalence is all the occurrences within a certain time period
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What is incidence?
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New cases that occur in a specified population over a particular time period
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What three things do you need to know to determine the incidence?
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number of new cases
time period they occur in number of animals total at the beginning |
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What do you do if animals leave the population during the time period you are calculating the incidence for?
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if they are removed NOT due to the disease, adjust the denominator by subtracting 1/2 of the number of animals removed
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What is the incidence rate?
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number of new cases over the sum of ALL individuals' times spent in the population at risk
(think cat months) |
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What is the mortality risk?
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number of animals that die of the disease over the suceptible population for that time period
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What is the case fatality risk?
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number of animals that die of the disease over the number of animals that got the disease in the time period
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What is a risk ratio?
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risk of the even occuring in the exposed group over the risk of the even occurring in the non-exposed group
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What values can be seen for risk ratios and what do they mean?
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R from 0<1 exposure was protective
R = 1 exposure had no effect R>1 exposure is associated with the disease |
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What is an odds ratio?
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ad/bc
the odds of the event occurring in the exposed group over the odds of the event occurring in the nonexposed group |
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How are odds ratios interpreted?
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same as RR
OR<1 is protective OR= 1 no correlation OR>1 correlated with disease |
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When is it appropriate to use a risk ratio?
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prospective studies (cohorts and clinical trials)
*DO NOT use with cross sectional and case control studies |
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When is it appropriate to use an odds ratio?
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can be calculated for anything, but mostly used for cross sectional and case control studies where RR cannot be used
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What p values are considered statistically significant? What does that mean?
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<= 0.05
it means there is less than a 5% likelihood that the results were due to chance |
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What is a confidence interval?
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indicates the reliability of an estimate
at the 95% confidence interval the true mean would fall within the interval 95 out of 100 times |
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How do you know if the confidence interval is significant?
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it must not include 1 (can be below or above, but not including)
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What is the absolute risk difference?
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incidence in the exposed group minus the incidence in the nonexposed group
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What is the attributable fraction?
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proportion of the disease burden among exposed animals that can be attributed to the exposure
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Why is the absolute risk difference calculated?
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removes background risk, since not all disease is usually caused by the exposure
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How is absolute risk difference interpreted?
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AR< 0 protective
AR= 0 no effect AR>0 exposure correlated with disease *CI should not include 0 to be significant in this case |
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How is the absolute risk difference different than a risk ratio?
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risk ratios do not remove background risk, so it doesn't give the magnitude of the effect of a causal factor
This is important in determining between two preventive programs, etc |
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how is the attributable fraction calculated?
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(RR-1)/RR
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What is the population absolute risk difference and how is it different than the absolute risk difference?
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Population absolute risk difference includes nonexposed animals, and thus is always lower than AR it is the incidence in the total population minus the incidence in the unexposed animals
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What is the number needed to treat?
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number of animals that need to be treated to prevent one case
1/AR *this is the same as number needed to harm, which is number of animals that must be exposed to cause disease in one |
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When do you use NNT versus NNH?
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if the exposure is occrelated with the outcome, use NNH
If the exposure is protective, use NNT |
