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PAS statistics > statistics > Flashcards

statistics Flashcards

1
Q

What does a case series do?

A

tracks subjects with a known exposure

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2
Q

What does a cross-sectional study do?

A

uses data fro a population at a specific point in time

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3
Q

What does a case-control study do?

A

2 existing groups differing in outcome are identified and compared based off causal attribute

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4
Q

What do cohort studies do?

A

you map a cohort and perform cross-sectional itnervals over time

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5
Q

What is the hierarchy of evidence?

A 
systematic reviews
critically-appraised topics
critically-appraise individual articles
RCTs
Cohort studies
Case-controlled studies
background information / expert opinion
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6
Q

what are confounders?

A

another variable associated with the outcome of interest and independent variable.

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7
Q

What is reverse causality?

A

because we are recruiting from the point of outcome, , the outcome may be affecting the variable

(oesophageal cancer may cause people to drink hot tea)

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8
Q

What do RCTs do?

A

similar people are randomly assigned to 2 (or more) groups to rest an intervention

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9
Q

what do systematic reviews do?

A

go through all the literature to identify every published (and some unpublished ones too) to answer the question we are posing

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10
Q

What are the 2 types of quantitative data?

A

continuous

discrete

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11
Q

What are the types of categorical data?

A

2 categories - binary

>2 categories - nominal / ordinal

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12
Q

What would you use to describe information if there are no outliers?

A

mean

standard deviation

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13
Q

What would you use to describe information if there are outliers?

A

Mean

interquartile range

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14
Q

What would a small SD suggest about a mean?

A

it is more useful

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15
Q

What sort of study design would be used to measure treatment effect?

A

RCT

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16
Q

What sort of study design would be used to measure exposure effect?

A

observational studies

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17
Q

In 10 000 control patients, 1 200 had a subsequent vascular event within 1 month…

What is the probability of a vascular event?

A

1 200 / 10 000 =

0.12

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18
Q

In 10 000 control patients, 1 200 had a subsequent vascular event within 1 month…

What is the percentage of those with a vascular event?

A

12%

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19
Q

In 10 000 control patients, 1 200 had a subsequent vascular event within 1 month…

What is the risk of a vascular event?

A

12 per 100

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20
Q

In 10 000 control patients, 1 200 had a subsequent vascular event within 1 month…

What are the odds of a vascular event happening?

A

1200 / 8800 =

0.14

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21
Q

What is relative risk?

A

probability of event on treatment / probability of event on control

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22
Q

What does a risk ratio of 1 suggest?

A

risk equal in intervention and control arm

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23
Q

What does a risk ratio > 1 suggest?

A

risk of outcome greater in treatment arm

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24
Q

What does a risk ratio < 1 suggest?

A

risk of outcome less in treatment arm

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25
Q

If you were looking at an exposure, what might you use instead of ‘treatment’ or ‘control?

A

risk in the exposed group

risk in the unexposed group

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26
Q

Name 3 ways you might interpret the relative risk

A

as a percentage increase or decrease in risk

as percentage of the risk in the other arm
risk in the intervention arm is RR times the risk in the control arm

27
Q

What is the number needed to treat?

A

The number of patients, who on average need to be treated to prevent one event that would otherwise occur

NNT is 1/absolute risk difference

28
Q

How do relative and absolute effects compare?

A
  • relative measures are commonly reported
  • relative measures can look large where an event is rare
  • absolute measures are less susceptible to misinterpretation

NNT is an effective way to communicate to lay population

29
Q

What are odds?

A

number with event / number without event

30
Q

What is an odds ratio?

A

the odds of event on treatment / the odds of event on control

31
Q

how does odds ratio compare to relative risk?

A

it’s a bit less intuitive

32
Q

What is a mean difference?

A

mean in group 1 - mean in group 2

33
Q

What might you use to measure treatment / exposure effects in continuous outcomes?

A

mean difference

34
Q

What might you use to measure treatment / exposure effects for a binary outcome?

A

relative measures (risk ratio / relative risk, odds ratio)

absolute measures (risk difference)

35
Q

What is sample variability?

A

the difference between truth (parameter) of the population and sample (statistic)

36
Q

What is a parameter?

A

a value referring to the population that we cannot know

37
Q

What is the standard error?

A
  • describes the variability in the means
  • tells us how accurate the mean of any particular sample is compared to the true population mean

the mean of 95% of the samples is within 2 SEs of each other

38
Q

What does a large standard error suggest?

A

the mean from each sample are likely to differ a lot and so could be an inaccurate representation of true population mean

39
Q

What does standard deviation do?

A

describes the variability in a sample.
essentially, how far, on average, a measurement is from the mean

about 95% of the sample have an observation within SDs of the sample mean

40
Q

How does SE suggest how certain we are about estimates?

A

Small study - large SE - uncertain

large study - small SE - more certain

41
Q

Which of SD or SE is usually bigger?

A

SD

42
Q

What does a confidence interval do?

A

suggests how accurate our estimate is likely to be

expresses a range of values which we are pretty sure the population parameter lies in (centre being the sample mean)

43
Q

What is the size of the confidence interval affected by?

A

1 - variation within the population
2 - sample size
3 - how confident do we want to be?

44
Q

What conditions might make the confidence interval really wide?

A

greater variation in population
small sample
we want to be really accurate

45
Q

What does a ‘95% confidence interval’ suggest?

A

95% of the time, it would contain the true mean

46
Q

How would you calculate the lower value of a confidence interval?

A

2 SEs below the mean/exposure effect

47
Q

How would you calculate the upper value of a confidence interval?

A

2 SEs above the mean/exposure effect

48
Q

What does a 95% CI for a RD above 0 (null value) suggest?

A

95% certain the risk in the treatment arm is greater than in the control arm

49
Q

What does a 95% CI for a RD below 0 (null value) suggest?

A

95% certain that the risk in the treatment arm is less that the control arm

50
Q

What does a 95% CI for a RD containing 0 (null value) suggest?

A

there is not enough evidence to say that the risk is different in the treatment arm compared to the control arm

51
Q

Name the 5 steps required to construct a hypothesis test

A 
specify null and alternative hypothesis
assume the null hypothesis is true and calculate test statistic
convert to a p-value
assess the evidence
interpret the result
52
Q

What does a null hypothesis suggest?

A

(H0) there is no difference

53
Q

What would an alternative hypothesis suggest?

A

(H1) there is a difference

54
Q

What is a p value?

A

the probability that the data could have arisen if the null hypothesis H0 were true

55
Q

What does a high p value tell us?

A

high chance of seeing the difference we’ve seen if the null hypothesis were true

56
Q

What does a smaller p value suggest?

A

greater evidence that the null hypothesis is not true

57
Q

How is statistical significance assessed?

A

p-value

58
Q

How is clinical importance assessed?

A

estimates

confidence intervals

59
Q

Is a 95% CI contains the null value, then what is the p value likely to be?

A

> 0.05

60
Q

If a 95% CI does not contain the null value, what is the p value likely to be?

A

<0.05

61
Q

If a 95% CI ends at the null value, what is the p value likely to be?

A

0.05

62
Q

What are the 3 categories used when comparing 2 CIs?

A

significant difference

unclear if significantly
different if boundaries overlap but means don’t

not significantly different if means overlap

63
Q

What is a forest plot?

A

a way of summarising results of a systematic review

64
Q

In a forest plot, what sort of studies are given a larger weight?

A

larger studies

PAS statistics (1 decks)

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