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OrthoBullets > Research and Stats > Flashcards

Research and Stats Flashcards

1
Q

Define positive predictive value

A

probability a patient with a positive test actually has a disease

PPV = TP / (TP + FP)

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2
Q

Define variance

A

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

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3
Q

define effect size

A

magnitude of difference in means between two groups

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4
Q

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

A

odds ratio

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5
Q

Define Type I error (alpha)

A

null hypothesis is rejected even though it is true

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6
Q

negative likelihood ratio: define and formula

A

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

(1-sensitivity)/specificty

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7
Q

what is a type II error?

A

beta.

false negative

detecting no difference when there is one

accepting a null hypothesis wrongly

typically set at 0.8

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8
Q

Describe Post-test odds of disease

A

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)

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9
Q

What does positive predictive value depend on?

A

prevalence of a disease

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10
Q

Define effect size

A

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

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11
Q

NNT formula and definition

A

NNT=1/ARR

number needed to treat to get one additional favourable outcome

alternatively, ARR=1/NNT

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12
Q

Define negative predictive value

A

probability a patient with a negative test actually has no disease

= TN / (FN + TN)

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13
Q

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

A

SENSITIVE tests with NEGATIVE results rule OUT the outcome

“SNNOUT”

SPECIFIC tests with POSITIVE results rule IN the outcome

“SPPIN”

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14
Q

define incidence

A

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

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15
Q

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

A

t-test

numeric data = t-test

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16
Q

How do you determine absolute risk reduction?

A

From NNT

NNT = 1 / ARR

ARR = 1 / NNT

or

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

*this was on a previous exam

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17
Q

test for comparing means of 3 or more continuous dependent variables

(each with categorical independent variables)

A

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

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18
Q

Define Type II error

A

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
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19
Q

sensitivity formula

A

TP/(TP+FN)

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20
Q

Student t-test

A

used to compare means of continuous data that is normally distributed

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21
Q

What does a relative risk > 1 mean?

A

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

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22
Q

test to see if two continuous variables are related or not

A

linear regression

e.g. comparing age to BP

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23
Q

Kaplan-Meier equation (in laymans terms)

A

The number of failures / total number still being followed

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24
Q

statistical tool in meta-analyses to detect publication bias

A

funnel plot

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25
Q

describe negative likelihood ratio

A

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

negative likelihood ratio = (1 - sensitivity) / specificity

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26
Q

define relative risk and give formula

A
  • 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)
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27
Q

Mann-Whitney or Wilcoxon rank sum tests

A

comparing means of non-continuous data

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28
Q

Describe Relative risk

A

risk of developing disease for people with known exposure

compared to

risk of developing disease for people without known exposure

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29
Q

incidence definition

A

number of new cases in a given time period

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30
Q

How do you compare categorical data?

A

chi-square test

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31
Q

What is the fisher exact test used for?

A

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

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32
Q

positive likelihood ratio; formula and definition

A

sensitivity/(1-specificity)

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

33
Q

What does blinding do?

A

minimizes observer bias

34
Q

Define positive likelihood ratio

A

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

positive likelihood ratio = sensitivity / (1-specificity)

35
Q

define likelihood ratio

A

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

36
Q

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

A

Type II error (beta)

37
Q

Define Number needed to treat

A

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

NNT = 1 / absolute risk reduction

38
Q

What does randomization do?

A

Decreases selection bias

39
Q

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

A

mann-whitney or wilcoxon sum rank test

40
Q

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

A

Having an adequate sample size based on power calculations

The use of appropriate levels of significance in hypothesis testing

41
Q

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

What can you do to prevent it?

A

Hypertension

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

42
Q

What is a funnel plot?

A

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

Is used to detect publication bias in meta-analyses

43
Q

Sensitivity

A

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

44
Q

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

A

Student t-test

45
Q

Define false negative

A

Patients with the disease but have a negative result

False negative rate = FN / (TP + FN)

= FN / everyone who is positive

46
Q

Describe Odds ratio

A

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)

47
Q

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

A

logistic regression

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

48
Q

define power (wrt stats) and give the formula

A

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

power=1-beta

49
Q

prevalence definition

A

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

50
Q

Specificity

A

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

51
Q

What does PPV depend on?

A

prevalence of a disease

52
Q

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

A

Sensitive

53
Q

PPV formula

A

TP/(FP+TP)

54
Q

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

A

retrospective studies (case-control)

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

55
Q

Define False positive

A

patients without the disease who have a positive test result

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

= FP / everyone who is negative

56
Q

Where do you get a relative risk ratio from?

A

Cohort studies

b/c you need incidence to calculate it

57
Q

test for comparing means of 2 continuous dependent variables

(each with categorical independent variables)

A

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)

58
Q

ANOVA

A

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

59
Q

test to compare 2 or more categorial dependent variables

(each with categorial independent variables)

A

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)

60
Q

specificity formula

A

TN/(FP+TN)

61
Q

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

A

Exclude them from the study

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

62
Q

define odds ratio and give formula

A

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)

63
Q

What is a bonferroni correction?

A

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

64
Q

What do cross-sectional studies aim to achieve?

A

Identify the prevalence of a condition

65
Q

what is a type I error?

A

alpha.

false positive

rejecting null hypothesis when it’s true

we typically set this at 0.05

66
Q

Describe a confidence interval

A

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

67
Q

Define alpha level?

A

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

typically set at 0.05

68
Q

What studies are generally reported as an odds-ratio?

A

Case-control study

69
Q

t-tests

mann-whitney sum rank tests

chi quare test

fischer exact test

are all types of what?

A

Statistical inference

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

70
Q

test to compare two categorical variables with small sample sizes

A

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

71
Q

What does NPV depend on?

A

prevalence of disease

72
Q

Define Power (stats)

A

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)

73
Q

define prevalence

A

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

74
Q

what is a bonferroni correction?

A

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

75
Q

What studies are generally reported as relative risk?

A

Cohort studies

76
Q

define Likelihood ratio

A

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

77
Q

NPV formula

A

TN/(FN+TN)

78
Q

What does matching in a study design do?

A

Minimizes confounders

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