Lectures Notes Flashcards
what are the two types of categorical data?
nominal and ordinal
what is nominal data? give examples
categorical data with no natural order
e.g. blood group, sex
what is ordinal data? give examples?
ordered categorical data.
e.g. pain severity, social class, grade of breast cancer
what are the two types of numerical data?
discrete and continuous
what is binary data?
a form of nominal categorical data, where there are only two categories
how might you display categorical data?
bar chart, pie chart
how might you display numerical data?
dot plot, stem and leaf, histogram, box and whisker
how do you calculate the mean of a data set?
total all the values, and divide by the number of values
how do you calculate the median of a data set?
order the values, median is the middle value.
if there is an even number of values, take the mean of the middle two values.
how do you calculate the mode of a data set?
the most common value observed
what’s the main advantage of using a median of a data set?
robust to outliers.
what’s the main advantage of using the mean of a data set?
uses all the data.
when would you use median vs mean?
symmetrical data = mean
skewed data = median
list the three main approaches to quantifying variability
range
interquartile range
standard deviation
what is the interquartile range of a data set, and how could you display it graphically?
the middle 50% of your data.
upper quartile - lower quartile.
box and whisker plot.
how do you calculate variance?
- draw a table
- calculate difference between observed value and mean for each value
- square each of these values
- calculate the total of the squared differences from mean
- divide this by n-1
(n= number of values)
how do you calculate standard deviation (SD)?
square root of variance
how many decimal places should you use when calculating SD?
usually 2 or 3 more decimal places than the original data
what is the relationship between mean and SD in Normally distributed data?
mean ± 1 SD covers 68% of data
mean ± 2 SD covers 95% of data
how do you calculate the ‘normal reference range’ of an investigation?
mean ± 2 SDs
what is the relationship between the mean and the median in Normally distributed data?
will be the same!
formula for risk
risk = no. events observed / number in the group
formula for risk difference
RD = risk (exposed group) - risk (unexposed group)
what’s the difference between risk difference and ABSOLUTE risk difference?
in ARD you ignore the sign - so it’s always expressed as a positive number, but it might represent an increase or decrease in risk