Week 9 Reading: Measuring and Summarising Data - Ben-Schlomo, Brookes Flashcards

1
Q

Medical Variable Types

2

A
  • Numerical variables
    • Continuous
    • Discrete
  • Categorical variables
    • Ordered
    • Unordered
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2
Q

Numerical Variable Types

2

A

Continuous = measurements on a continous scale
Discrete = counts
generally treated same way

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3
Q

Categorical Variables

=, 2

A

= variables that take nonnumerical values and refer to categories of data
- Unordered = class observations into named groups
- Ordered

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4
Q

Continuous Numerical Variables

A

= measurements on a continuous scale
e.g. height, haemoglobin, systolic blood pressure

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5
Q

Discrete numerical variables

A

= counts
e.g. no. children in a family, no. asthma attacks in a week

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6
Q

Unordered Categorical Variables

A

= class observations into named groups
e.g. ethnic group, marital status, disease categories
Binary/dichotomous = special case, class observations into 2 groups usually indicating presence or absence of a characteristic

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7
Q

Ordered Categorical Variables

A

= rank observations according to an ordered classification
e.g. social class, severity of disease (mild, moderate, severe), stages in development of cancer
often in epidemiological studies a variable is measured as numerical and then categorised
e.g. height measured then <5ft, 5ft-5ft 5in, 5ft 5in-6ft, >6ft

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8
Q

Binary/Dichotomous Unordered Categorical Variables

A

= special case of unordered categorical variables classing observations into 2 groups, generally indicating presence or absence of a charecteristic
e.g. presence vs absence of chest pain, smoker vs non-smoker, vaccinated vs unvaccinated

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9
Q

Measures of Central Tendency

=, 3, sub 3

A
  1. Mean = sum of all values in a set divided by no. values
  2. Median = middle value when set arranged in order. If even no., take mean of 2 middle values
  3. Mode = most frequently occuring value/peak on frequency distribution histogram
    • Unimodal = single mode/peak
    • Bimodal = 2 modes/peaks
    • Multimodal = >1 mode/peak
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10
Q

Measures of Variability

=, 3

A

Variability = extent to which values of a variable in a distribution are spread
1. Range = difference between largest and smallest values
2. Interquartile range = range between quartiles
- Quantiles = divisions of set of values into equal, ordered subgroups
- can have tertiles, quartiles, quintiles, deciles, centiles etc.
3. Standard Deviation (SD) = spread of observations about the mean, based on differences/deviations from mean
- differences are squared to remove effect of sign
- SD is calculated as square root of sum of squared deviations divided by no. deviations minus 1
- SD squared = variance

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11
Q

Normal/Gaussian Distribution

A
  • mean, median and mode aree identical, define location of curve
  • SD determines shape of curve
    • Small SD –> tall, narrow
    • Large SD –> short, wide
  • use mean and SD to determine proportion of data lying between 2 variables, rules apply regardless of values of mean and SD:
    1. - 68.3% lie within 1 SD of mean
    • 95.4% lie within 2 SD of mean
    • 99.7% lie within 3 SD of mean
      1. Because of symmetry:
    • 15.85% lie above 1 SD above mean or below 1 SD below mean
    • 2.3% lie above 2 SD above mean or below 2 SD below mean
      1. 95.0% observations enclosed between mean - 1.96 x SD to mean + 1.96 x SD
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12
Q

Case Series

A

= describing frequency of characteristics in a patient sample

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13
Q

Proportion

=, 2

A

= (number with disease)/(total number)
can be x 100 to make it a percentage
- Prevalence = proportion (or %) with disease at a particular point in time
- Cumulative incidence/Risk = proportion (or %) of new cases of disease occuring in a specified time period

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14
Q

Prevalence

=, =

A

= proportion (or %) with disease at a particular point in time
Prevalence = (no. with disease at particular time)/(total no. population at that time)

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15
Q

Cumulative Incidence/Risk

=, =

A

= proportion (or %) of new cases of disease occuring in a specified time period
Risk = (no. new cases in a period)/(no. initially free of disease)

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16
Q

Incidence

=, =

A

= how fast new cases are occuring
Incidence rate = (no. new cases)/(total no. x time interval)

17
Q

TPP

=, 2

A

= Time Place Person, how epidemiologists describe disease patterns
useful for:
- planning healthcare services
- generating aetiological hypotheses

18
Q

Potential Explanations for Increased or Decreased Risk

6

A
  1. Chance = random fluctuations
  2. Ascertainment = change in diagnostic techniques
  3. Demography = change in age distribution of population
  4. Coding = changes in rules by which mortality is coded (ICD). Demonstrate with bridge coding = comparing new rates using old rules
  5. Treatment effects = new medical therapies can increase or decrease disease frequency or mortality
  6. True changes in incidence = true increase or decrease in incidence, implies risk factors
19
Q

Bridge Coding

A

= comparing new rates using old coding rules

20
Q

Null hypothesis

A

= assumption of no association between disease and outcome

21
Q

Difference in Means

=, =

A

= measure the association between and exposure and outcome when the exposure is dichotomous/binary and the outcome numerical
difference in means = mean in exposed - mean in unexposed

22
Q

Risk Difference/Attributable Risk

=, =, +, 0, -

A

= measure the association between exposure and outcome when both are dichotomous/binary
risk difference = risk among exposed - risk among unexposed
+ve value indicates increased risk
0 indicates no difference
-ve indicates reduced risk

23
Q

Absolute measures of association

3

A
  • difference in means
  • risk difference/attributable risk
  • population attributable risk
    have units
24
Q

Population Attributable Risk

=, =

A

= measures how much of overall population risk is attributable to an exposure
population attributable risk = overall risk - risk among unexposed

24
Q

Relative Measures of Association

4

A
  • Risk ratio/relative risk
  • Odds of disease
  • Odds ratio
  • Hazard Ratio
    unitless
25
Q

Risk Ratio/Relative Risk

=, =, 3

A

= how much more likely the outcome is among those exposed compared to unexposed, used when both outcome and exposure are dichotomous/binary
risk ratio = (risk in exposed individuals)/(risk in unexposed individuals)
- >1 = increased risk
- 1 = no difference in risk
- <1 = reduced risk

26
Q

Odds of Disease

=

A

odds of disease = (no. with disease)/(no. without disease)

27
Q

Odds Ratio

A

odds ratio = (odds of disease in exposed individuals)/(odds of disease in unexposed individuals)
= (odds of exposure in individuals with disease)/(odds of exposure in individuals without disease)
= (d1/d0)/(h1/h0)
= (d1 x h0)/(d0 x h1)
where d1 = no. exposed in disease group, d0 = no. unexposed in disease group, h1 = no. exposed in healthy group, h0 = no. unexposed in heathy group
second form of odds ratio is used in case-control studies

28
Q

Hazard ratio

=

A

= used for time to event data (as in survival analysis)

29
Q

Reference Range

A

= determines the proportion of data lying between any 2 values

30
Q
A