Equations Flashcards
In a two by two table, what is on the top and what is on the side.
top, x = disease = the d
side, y = test or exposure
Starting from the top left to the bottom right, what is the order of true positives and true negatives in a 2x2 table?
TP, FP FN, TN = a,b c,d
Sensitivity equation and blank rate
a/a+c
true positive rate
snaac
Specificity equation and blank rate
d/d+b
true negative rate
Positive predictive value
a/a+b
Negative predictive value
d/d+c
Odds ratio equation and used in what type of study? Mnemonic?
a/c
—– = ad/bc
b/d
Or add because
used in case-control studies
Odds ratio interpretation/ used in what type of study?
OR > 1 Means that exposure is associated with higher odds of the outcome
OR < 1 Means that exposure is associated with lower odds of the outcome
OR = 1 means No difference between exposure and Outcome
Used in case control studies
Relative risk equation and used in what type of study?
c/c+d
Used in cohort studies
Relative risk meaning: RR < > & =
RR>1 Means increased risk of outcome
RR<1 Means decreased risk of outcome
RR=1 Means no risk of of outcome
Relative risk reduction
1 - relative risk
=
1 - ((a/a+b)/(c/c+d))
Absolute risk = probability of an event ____/ given ____ period
probability of an event occurring/ given time period
Absolute risk reduction
c/c+d - a/a+b
Number needed to treat
1/absolute risk reduction
1/(c/c+d - a/a+b)
Attributable risk/ Absolute risk increase
a/a+b - c/c+d
Number needed to harm
1/attributable risk
1/(a/a+b - c/c+d)
Incidence
population at risk
Prevalence
population at risk
Positive likelihood ratio
sensitivity/ 1 - specificity –> ((a/a+c)/(1 - d/d+b))
=
non diseased people with a + test result
Negative likelihood ratio
1 - sensitivity/ specificity –> ((1 - a/a+c)/(d/d+b))
=
non diseased people with a - test result
Hazard ratio =
when are Hazard ratios used? What are they similar to?
Hazard in the control group
Hazard ratios are interpreted similarly to relative risk and are used in cohort studies. You use confidence intervals to interpret them. A confidence interval that includes one is not statistically significant.
Interpret a hazard ratio of .5, 1, 2
Hazard in intervention group
HR= —————————————————–
Hazard in control group
HR = 0.5: at any particular time, half as many patients in the treatment group are experiencing an event compared to the control group.
HR = 1: at any particular time, event rates are the same in both groups,
HR = 2: at any particular time, twice as many patients in the treatment group are experiencing an event compared to the control group.
Attack rate =
total population at risk
What is a foodborne attack rate
A food-specific attack rate is the number of persons who ate a specified food and became ill divided by the total number of persons who ate that food
Define positive likelihood ratio in words
Ratio representing the likelihood of having the disease given a positive test result
Negative likelihood ratio, define in words
Ratio representing the likelihood of having a disease given a negative result
Give the three percents associated with 1, 2, and 3 standard deviations away from the mean
68
95
99.7
