1 - Intro to Stats Flashcards
What are the 3 common scales of measurement for variables in medicine?
- Nominal
- Ordinal
- Numeric (interval or ratio)
Describe nominal measurement
- Simplest b/c data fits in categories in no particular order
- No actual measurement
- Often dichotomous or binary (yes/no, male/female)
- Can be multiple categories
- Generally described in percentages or proportions
Describe ordinal measurement
- Inherent order to the categories
- Summary statistic (median)
- Often used in assessment of pt risk
- Difference between 2 adjacent categories isn’t the same throughout the scale
Describe numerical measurement
- Differences have meaning on numerical scale
- 2 types of numerical scales - interval and ratio
Describe the difference between interval and ratio
- Interval - difference between any pair of levels is the same (ex: temperature 10-15 = 20-25), but no meaningful zero value
- Ratio - interval scale w/ meaningful zero value (ex: time)
What are the 3 types of variables? Give examples of each
- Continuous (age, time)
- Discrete (number of houses on a street)
- Summary statistics (mean and SD)
What are the 3 measures of middle?
Mean, median, and mode
Describe mean
- Arithmetic average
- Mean = sum of x/n (x = individual observation; n = number of observations)
- Used w/ numerical variables; shouldn’t be used w/ ordinal variables (but often is)
Describe median
- Middle observation
- Arrange the observations from smallest to largest; count and find the middle
- For odd number of observations - median is the middle observation
- For even number - median is average of the values on either side of the middle
Describe mode
- Value that occurs most frequently
- Data can have more than 1 mode (bimodal distribution)
Which measure of middle should be used in skewed distributions?
- Use mean in symmetric (normal) distributions
- Use median for ordinal data or numerical data that is skewed (mean very sensitive to extreme values in small datasets)
What are the 4 measures of spread (dispersion)?
- Range
- Standard deviation/ variance
- Percentiles
- Interquartile range
Describe range
- Difference between the smallest and largest observation
- Minimum and maximum may also be given
Describe the standard deviation formula
- s = square root of [(sum of x - /x) ^ 2 / (n - 1)]
- x = value
- /x = mean
- n = sample size
What is variance?
Sum of x - /x before square root is taken