critical numbers

Question 1

what is prevalence probability?

Answer 1

probability of having a disease at a given point in time

Question 2

what is incidence probability?

Answer 2

probability of getting a disease during a specified poit in time

Question 3

what is the length-time bias?

Answer 3

conditions with a longer duratoon are more likely to be captured in prevalence

Question 4

if risk of X given: drug A = 0.6 and drug B = 0.2 what is the risk difference?

Answer 4

0.6-0.2 = 0.4 (40%)

Question 5

if risk of X given: drug A = 0.6 and drug B = 0.2 what is the risk ratio?

Answer 5

=3

3 times the risk

Question 6

if risk of X given: drug A = 0.6 and drug B = 0.2 what is the relative risk difference?

Answer 6

200%

((a-b)/b) *100 = 200%

Question 7

relative risk ____ the risk

Answer 7

relative risk exaggerates the risk, they do not indicate a baseline

Question 8

what is number needed to treat/harm?

Answer 8

indicates the potential benefit of a clinical intervention

Question 9

what type of study uses routinely collected data to show trends and generate hypotheses?

Answer 9

ecological

Question 10

what type of study divides a population into those with the disease and those without and collects data at a single point in time?

Answer 10

cross-sectional study

Question 11

what type of study matches people with a disease to those without for age, sex, habits, class etc.?

Answer 11

case-control study -retrospective

Question 12

what type of study studies a population to see if they’re exposed to the agent in question and if they develop the disease?

Answer 12

cohort study - prospective

Question 13

give 3 advantages of an ecological study?

Answer 13

1) uses routine date and so is quick and cheap
2) few ethical issues
3) useful for generating hypotheses

Question 14

give 3 disadvantages of an ecological study

Answer 14

1) cannot show causation
2) inconsistency in data presentation
3) bias - variation in diagnostic criteria

Question 15

give 4 advantages of a cross-sectional study

Answer 15

1) can give rapid insight into events within a population
2) few ethical issues
3) good for generating hypotheses
4) quick and cheap

Question 16

give 3 disadvantages of a cross-sectional study?

Answer 16

1) prone to bias
2) no time bias
3) could be reporting medical oddities

Question 17

give 3 advantages of a case-control study

Answer 17

1) quick - results can be obtained quickly (retrospective)
2) cheap
3) usually a small number of people required to produce statistically significant results

Question 18

give 3 disadvantages of a case-control study?

Answer 18

1) retrospective date may be unreliable - selective memory
2) shows association but not causation
3) prone to selection and information bias
4) cannot calculate incidence

Question 19

give 3 advantages of a cohort study?

Answer 19

1) can calculate incidence and so find relative and abs risk
2) reduced chance of bias - exposure measured before disease develops
3) can distinguish causes from associated factors

Question 20

give 3 disadvantages of a cohort study?

Answer 20

1) expensive - long time and large population
2) causation cannot be calculated - control study is needed for this
3) often difficulties with follow-up

Question 21

give 3 advantages of a RCT?

Answer 21

1) confounders are equally balanced
2) blinding minimises bias
3) statistical tests are easier when confounders are minimised

Question 22

give 4 disadvantages of a RCT

Answer 22

1) expensive - large populations
2) poor compliance can mean statistical tests lose their power
3) volunteer bias
4) ethical difficulties in withholding treatment from control groups

Question 23

what does presence of bias in a set of results imply?

Answer 23

the results of the study are not accurate

Question 24

will the standard error in a set of results be big or small if the precision is good?

Answer 24

there will be small standard error

Question 25

what are confounding factors?

Answer 25

factors that can affect the validity of a study | they may be responsible for the results seen

Question 26

what should a good study have?

Answer 26

randomisation of participants to interventions (reduces bias/confounding) show causation rather than association have outcome measures (results) for atleast 80% of the population

Question 27

define systematic review

Answer 27

systematic methods are used to identify, select and analyse relevant research in order to answer a specific question

Question 28

give 2 examples of non-random sampling?

Answer 28

convenience sampling | purposive sampling

Question 29

what is number needed to treat?

Answer 29

the number of patients that need to be treated in order to have an impact on one person

Question 30

is a test significant if the 95% confidence interval contains zero?

Answer 30

no it is not significant

Question 31

how can you calculate the NNT?

Answer 31

1/risk difference

Question 32

what is nominal data?

Answer 32

a qualitative classification of data - a naming system

Question 33

what is ordinal data?

Answer 33

when data has numerical scores existing in order

Question 34

what is binary data?

Answer 34

data that can only take one of 2 possible states e.g. daed or alive

Question 35

what is discrete data?

Answer 35

finite number of values e.g. number of people living in a house

Question 36

what is continuous data?

Answer 36

data that can take any value e.g. height

Question 37

what is linear regression used for?

Answer 37

estimating mean differences between groups

Question 38

what is logistic regression used for?

Answer 38

binary outcomes, it models and odds ratio

Question 39

what is poisson regression used for?

Answer 39

it models rate ratios

Question 40

what is cox regression used for?

Answer 40

models hazard ratios

Question 41

define morbidity

Answer 41

suffering from a disease

Question 42

if the prevalence of a disease is low, what affect does this have on false negatives ?

Answer 42

number of false negatives will decrease | NPV will increase and PPV will decrease

Question 43

what is association?

Answer 43

a statistical link between exposure and disease

Question 44

what is causation?

Answer 44

a statistical link where a disease is directly caused by the exposure

Question 45

what percentage of all data in a sample will fall into one SD limit?

Answer 45

68%

Question 46

what percentage of all data will fall into two SD limits?

Answer 46

95%

Question 47

what does a low standard error mean?

Answer 47

low variability from sample to sample better precision in estimating mean low in a large sample size estimates the variability

Question 48

what does a 95% confidence interval mean?

Answer 48

the range of values that you can be 95% confident contains the true mean of the population

Question 49

what is a null hypothesis?

Answer 49

a statement to be tested of no difference or association typically aim to prove this hypothesis is not true

Question 50

statistical significance _____ mean clinical significance

Answer 50

statistical significance doesn't mean clinical significance

Question 51

what does crude mean in statistics?

Answer 51

where confounding variables aren't accounted for

Question 52

what does adjusted mean?

Answer 52

where confounding variables are accounted for

Question 53

how can chance be reduced?

Answer 53

ensuring a sufficiently large sample using confidence intervals and p-values

Question 54

how can confounders be prevented at design stage?

Answer 54

randomisation restriction matching - where control group matches the confounders in the case group

Question 55

how can bias be prevented?

Answer 55

ensuring an appropriate selection of participants | ensuring data is collected and measured correctly

Question 56

what are the 2 types of bias?

Answer 56

selection bias - problem with study population | information bias - problem with information provided

Question 57

how can information bias occur?

Answer 57

researcher - observer bias participant - recall/respondent bias instruments - wrongly calibrated instruments