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

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- 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

positive likelihood ratio; formula and definition

sensitivity/(1-specificity)

how likelihood of a disease is changed by a positive test result

What does blinding do?

minimizes observer bias

Define positive likelihood ratio

describes how the likelihood of a disease is changed by a positive test result

positive likelihood ratio = sensitivity / (1-specificity)

define likelihood ratio

likelihood a result is expected in someone with the disease compared to likelihood the result is expected in a normal person

An underpowered study is prone to what type of statistical error?

Type II error (beta)

Define Number needed to treat

number of patients that must be treated in order to achieve one additional favourable outcome

NNT = 1 / absolute risk reduction

What does randomization do?

Decreases selection bias

What test do you use to compare 2 means with non-parametric (non-normal) data?

mann-whitney or wilcoxon sum rank test

Name 2 ways of minimizing the effects of chance in study design

Having an adequate sample size based on power calculations

The use of appropriate levels of significance in hypothesis testing

What is an independent risk factor for postop MI post total joint arthroplasty?

What can you do to prevent it?

Hypertension

Administration of beta blockers for 7 days decreases risk of cardiac ischaemic events and in-hopsital deaths

What is a funnel plot?

Is a simple scatter plot of the intervention effect estimates from individual studies against some measure of each study’s size or precision

I**s used to detect publication bias in meta-analyses**

Sensitivity

probability that test results will be positive in patients *with* the disease

= TP / (TP + FN)

= people with the disease who tested positive / everyone with the disease

What test do you use to compare 2 means with normal (parametric) data

Student t-test

Define false negative

Patients with the disease but have a negative result

False negative rate = FN / (TP + FN)

= FN / everyone who is positive

Describe Odds ratio

probability of having a risk factor if one has a disease

obtained from case control studies (retrospective studies)

OR = (odds of developing disease in exposed patients) / (odds of developing disease in unexposed patients)

test to show how a change in a variable (categorical or continuous) affects another variable (categorical, dichotomous)

logistic regression

e.g. seeing if body weight is related to presence or absence of lung cancer

define power (wrt stats) and give the formula

estimate of the probability a study can detect a true effect of the intervention

power=1-beta

prevalence definition

number of cases of something at a single point in time for a given population

Specificity

probability that the test result will be negative in patients *without* the disease

= TN / (FP + TN)

= people without disease who tested negative / everyone who *doesn't* have the disease

What does PPV depend on?

prevalence of a disease

In a screening test, you want it to be highly _______________

Sensitive

PPV formula

TP/(FP+TP)

What kind of studies do you get odds ratio's from?

retrospective studies (case-control)

b/c it is the proability of having a risk factor given a person has the disease

Define False positive

patients without the disease who have a positive test result

false positive rate = false positives / (FP + TN)

= FP / everyone who is negative

Where do you get a relative risk ratio from?

Cohort studies

b/c you need incidence to calculate it

test for comparing means of 2 continuous dependent variables

(each with categorical independent variables)

student t-test

e.g. comparing average BP (the continuous dependent variable) in a group of smokers vs. nonsmokers

(smoking is the categorial independent variable)

ANOVA

compare means of 3 or more independnet groups in normally distributed data

test to compare 2 or more categorial dependent variables

(each with categorial independent variables)

chi-square

e.g. comparing # of ppl with ecoli who ate burgers with #ppl with ecoli who didnt eat burgers

(# of ppl is categorical)

i.e. if you can fit your data into a 2x2 table, use a chi-square test

(ate burger, didnt eat burger vs. ecoli, no ecoli)

specificity formula

TN/(FP+TN)

In a Kaplan-Meier analysis, what do you do with the patients that are lost to followup

Exclude them from the study

They are assumed to be similar to the patients still in the study

define odds ratio and give formula

probabilyt of having a risk factor given you already have the disease

OR=(odds of getting disease when exposed)/(odds of getting disease if unexposed)

What is a bonferroni correction?

post-hoc statistical correction made to P values when several dependent or independent statistical tests are being performe dsimultaneously on a single data set

What do cross-sectional studies aim to achieve?

Identify the prevalence of a condition

what is a type I error?

alpha.

false positive

rejecting null hypothesis when it's true

we typically set this at 0.05

Describe a confidence interval

The Interval that will include a specific parameter of interest, if the experiement is repeated

Define alpha level?

probability of a type I error occuring (reject null when its true)

typically set at 0.05

What studies are generally reported as an odds-ratio?

Case-control study

t-tests

mann-whitney sum rank tests

chi quare test

fischer exact test

are all types of what?

Statistical inference

used to test specific hypotheses about associations or differences among groups of subjects/sample data

test to compare two categorical variables with small sample sizes

fisher exact test (similar idea to chi-square but with small sample sizes)

for samples <5 or total of all cells <50 (in your 2x2 table) - see chi-square card

What does NPV depend on?

prevalence of disease

Define Power (stats)

an estimate of the probability a study will be able to detect a true effect of the intervention

power = 1 - (probability of a type II, or beta, error)

define prevalence

total number of cases of a disease present in a location at any time point

what is a bonferroni correction?

post-hoc statistical correction to P-values when several tests are performed simultaneously on a single data set

What studies are generally reported as relative risk?

Cohort studies

define Likelihood ratio

likelihood that a given test result would be epxected in a patient with the target disease

compared to

likelihood that the same result would be expected in a patient without the target disease

NPV formula

TN/(FN+TN)

What does matching in a study design do?

Minimizes confounders