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71 Cards in this Set

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