# Research and Stats Flashcards

Define positive predictive value

probability a patient with a positive test actually has a disease

PPV = TP / (TP + FP)

Define variance

an estimate of the variability of each individual data point from the mean

define effect size

magnitude of difference in means between two groups

case control studies (retrospective) typically generate what statistical measure?

odds ratio

Define Type I error (alpha)

null hypothesis is rejected even though it is true

negative likelihood ratio: define and formula

how the likelihood of a disease is changed by a negative test result

(1-sensitivity)/specificty

what is a type II error?

beta.

false negative

detecting no difference when there is one

accepting a null hypothesis wrongly

typically set at 0.8

Describe Post-test odds of disease

post-test probability = (pretest probabililty) X (likelihood ratio)

- likelihood ratio = sensitivity / (1 - specificity)
- pre-test odds = pre-test probability / (1 - pre-test probability)

post-test probability = post-test odds / (post-test odds + 1)

What does positive predictive value depend on?

prevalence of a disease

Define effect size

magnitude of the difference in the means of the control and experimental groups in a study with respect to the pooled standard deviation

Effect sizes are normally used for continuous variables in contrast to relative risk reduction which is used for dichotomous variables

NNT formula and definition

NNT=1/ARR

number needed to treat to get one additional favourable outcome

alternatively, ARR=1/NNT

Define negative predictive value

probability a patient with a negative test actually has no disease

= TN / (FN + TN)

a highly ____ test with a (neg/pos) result can rule (in/out) the outcome of interest

SENSITIVE tests with NEGATIVE results rule OUT the outcome

“SNNOUT”

SPECIFIC tests with POSITIVE results rule IN the outcome

“SPPIN”

define incidence

number of newly reported cases of a disease during a given time period

What test do you use to compare 2 independent means from numeric data?

t-test

numeric data = t-test

How do you determine absolute risk reduction?

From NNT

NNT = 1 / ARR

ARR = 1 / NNT

or

**ARR = (risk in control group - risk in experimental group)**

*this was on a previous exam

test for comparing means of 3 or more continuous dependent variables

(each with categorical independent variables)

ANOVA

this is essentially a t-test for 3 or more groups

ANOVA on 2 groups will give the same result as a t-test

Define Type II error

Beta error

a false negative difference that can occur by:

- detecting no difference when there is a difference
- accepting a null hypothesis when it is false and should be rejected

sensitivity formula

TP/(TP+FN)

Student t-test

used to compare means of continuous data that is normally distributed

What does a relative risk > 1 mean?

incidence of the outcome is greater in the exposed/treated group

test to see if two continuous variables are related or not

linear regression

e.g. comparing age to BP

Kaplan-Meier equation (in laymans terms)

The number of failures / total number still being followed

statistical tool in meta-analyses to detect publication bias

funnel plot

describe negative likelihood ratio

describe how the likelihood of a disease is changed by a negative test result

negative likelihood ratio = (1 - sensitivity) / specificity

define relative risk and give formula

- risk of developing disease with exposure compaerd to risk of developing disease without exposure
- risk of disease with exposure=(have exposure and disease)/(all with exposure)
- risk of disease without exposure=(no exposure but have disease)/(all without exposure)
- RR=(risk of disease with exposure)/(risk of disease without exposure)

Mann-Whitney or Wilcoxon rank sum tests

comparing means of non-continuous data

Describe Relative risk

risk of developing disease for people with known exposure

compared to

risk of developing disease for people *without* known exposure

incidence definition

number of new cases in a given time period

How do you compare categorical data?

chi-square test

What is the fisher exact test used for?

Comparing proportional or categorical data when sample sizes are small or number of occurences in a group is low