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71 Cards in this Set
- Front
- Back
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4 Biases that can threaten the validity of a study:
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1. Selection bias
2. Measurement bias 3. Confounder bias 4. Simply chance |
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What is an event rate?
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Incidence
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CER
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control event rate
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EER
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experimental event rate
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How do you calculate EER in an RCT?
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# of exposed w/ disease
----------------------- total exposed |
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How do you calculate CER in an RCT?
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# of nonexposed w/ disease
-------------------------- total control (nonexposed) (convert denom to 10,000) |
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How do you calculate RR?
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Relative Risk = EER/CER
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How do you calculate the percent increase in risk?
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By calculating the Relative Risk Difference
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How to calculate Relative Risk Difference:
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ARD/CER
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What is ARD?
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Absolute Risk Difference
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How to calculate Absolute Risk Difference:
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EER - CER
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How do you calculate NNT?
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number needed to treat = 1/ARD
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What is the NNT?
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The number of patients who would need to recieve the intervention in order for one event to occur.
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What does the NNT allow you to do?
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Compare outcomes and treatments directly
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What is the stipulation for RCT to be able to generate a Relative Risk Ratio?
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The outcome must be dichotomous
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What do you do if the variable outcome is not dichotomous, ie is continuous?
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Look at the mean change in the variable and compare mean differences.
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3 measures of central tendency:
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-Mean
-Median -Mode |
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Mean
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average
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Median
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middle value
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Mode
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most common value
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2 measures of dispersion:
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-Standard deviation
-Range |
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Standard deviation:
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variation around the mean
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Range:
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highest to lowest value
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What is the measure of effect for continuous variables?
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Mean difference
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What are the measures of effect for Dichotomous variables (4)?
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1. Relative Risk
2. Risk difference 3. Odds ratio 4. Hazard ratio |
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How should you always err in clinical research?
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On the conservative side - the cause is innocent until proven guilty for causing the effect.
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What is the difference between Sample and Population?
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Sample is a subset of the population studied, the latter consisting of everyone in whom you're interested.
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N:
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the number of persons in the sample. can't believe you made this card.
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When formulating a research question, what do we begin with?
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-Null hypothesis
-Alternative hypothesis |
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What is the null hypothesis?
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That there is no difference between exposed/unexposed groups. (presume innocence)
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What is the Alternative Hypothesis?
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That there really is a difference.
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What are the four possibilities for your hypotheses at the end of a study?
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1. Accept the Null and it's true
2. Accept the alternative and it's true 3. Accept the alternative but it's false 4. Accept the null but it's false |
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What type of error is it when you accept the alternative and it's false?
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Type I
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What type of error is it when you accept the null and it's false?
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Type II
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Which type of error is worse; Type I or Type II? Why?
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Type I - because you say there is a difference in exposed vs unexposed, caused by the factor, but it's not really true - totally not erring on the conservative side.
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What is "alpha"?
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The threshold of reasonable doubt we will accept to make a type I error
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What is the usual value for alpha?
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0.05 (5%)
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What happens if alpha is greater than 5%?
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The results are not statistically significant.
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What is the difference between a Statistic and Parameter?
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Parameter - a characteristic of the population. (true mean)
Statistic - a characteristic of the sample (SD, Event Rate (CI) |
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What do we hope to do in RCTs?
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Generalize the statistics generated by the sample, to describe the parameters of the population of interest.
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What would a Type II error likely be due to?
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Small sample
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What is Beta?
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The probability of a type II error.
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What is the usual value of Beta?
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0.20 (20%)
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What is Power?
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1-Beta = .80 (80%)
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What does Power refer to?
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The power of a study to detect a difference.
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When flipping a fair coin, what are the odds that you would get heads 10 times in a row, randomly?
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Less than 0.1%
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What is the null hypothesis here? Alternative?
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Ho = 10 heads would land in a row, randomly
Ha = 10 heads would land in a row not randomly, but due to some external influence. |
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Since the probability (p) of getting heads 10X in a row randomly is soooo low, what is the conclusion?
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Reject Ho and Accept Ha.
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If a new drug lowers BP by 10 mm Hg lower than the old drug, how do you tell if the new drug truly is better?
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By doing a statistical test to tell if the probability (p) of getting this difference by random chance alone is <0.05
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How do you know if the new drug is better?
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If the p value is low - it's not probalby just random luck that the new drug lower BP by 10mmHg.
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4 Things necessary to calculate sample size:
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1. Set alpha/beta ahead of time
2. Choose statistical tests 3. Make assumptions about what you expect - effect size, variability 4. Calculate how many subjects would be needed to recruit these levels |
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What MUST BE in order for results to be significant?
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P must be less than alpha!!!
(<0.05) |
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What CAN'T you do with P's?
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Compare P's from 2 studies - say one study is more significant than another because the P value was lower..
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P
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Probability of finding due to chance alone is small (alpha)
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Power
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Power to prove a relationship does not exist (1-beta)
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Power is highly dependent on:
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sample size
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What is a Confidence Interval?
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A description of statistical significance that gives more information on the size and accuracy of the effect measure than P does.
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What makes the best CI?
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-Narrower
-Doesn't cross null |
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What does it mean if CI crosses the null?
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It is not significant.
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What is the null for mean differences (blood pressure)?
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0
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What is the mean for relative risks? (dichotomous variables)
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1
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What is the CI calculated from?
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The mean
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Formula for the 95% CI
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mean +/- 1.96
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4 Components in the PICO model for RCT studies:
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-Population
-Intervention -Comparison -Outcome |
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What does the RCT begin with?
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The hypothesis that includes the PICO format
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2 types of validity that we look for in an RCT:
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-Internal
-External |
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What is internal validity?
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That the study results are free from bias and error.
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What is external validity?
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That the study results can be generalized - sample results attributable to the population.
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How should an RCT investigator assign subjects to control vs expirimental groups?
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RANDOMLY according to number sequences.
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Why is randomization important?
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It limits the effects of confounders.
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What is "intention-to-treat"?
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Keeping subjects in the group (case vs control) to which they were randomized at the beginning of the trial.
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