Lecture 2 - Central Tendency, Variability, Stem & Leaf, Density Plots Flashcards
How do you calculate the Mode?
Find the most frequently occurring score
How do you calculate the Median?
Mdn = (n+1) / 2 Median = Number of scores plus 1, divided by 2
How do you calculate the Mean?
X̄ = (Σxi) / N Mean = The sum of each individual score, divided by the number of scores
How and why would you use a “trimmed mean”?
How: Cut an equal amount of scores from each end of the data set and then find the mean.
Why: To eliminate extreme values that may be distorting the data
What is the most appropriate measure of central tendency to use for nominal data?
The mode
What are the most appropriate measures of central tendency to use for ordinal data?
The mode or median
What are the most appropriate measures of central tendency to use for interval/ratio data?
The mode, median or mean
List 3 reasons why the Mean is useful.
- It takes every score into account
- It can be used algebraically
- It gives stable estimates of the population mean
When is it best to NOT use the mean?
When the data is skewed / there are extreme scores, because the mean is more sensitive to outliers than the mode or median.
What is the Interquartile Range?
The middle 50% of scores
What is the Range?
Ra = Highest Score - Lowest Score
What is Quartile 2 also known as?
The Median
How do you calculate the Interquartile Range?
IQR = Q3 - Q1
How do you calculate the value for Quartile 1?
(n+1) / 4 = the position of the Q1 value in the range of scores
How do you calculate the value for Quartile 3?
3 ( (n+1) / 4 ) = the position of the Q3 value in the range of scores
If you calculate the deviation from the mean for every score in a sample and add them all together, what will the total always be?
0
How do you calculate the sample variance of a set of scores?
s² = Σ(X-X̄)² / N-1
The sum of the squared deviations from the mean divided by N - 1
How do you calculate the sample standard deviation?
SD = Square root of the sample variance
Why do we use N-1 rather than just N in determining the sample variance and sample standard deviation?
Because this gives us the most unbiased estimation of the population parameters.
What is a stem and leaf diagram?
A structured and ordered way of displaying the data where the tens column become the “stem” and the units become the “leaves”
eg. for the data 1,1,3,4,6,12,12,15,23,26,26
0 | 1,1,3,4,6
1 | 2,2,5
2 | 3,6,6
Why is a stem and leaf diagram better than a histogram?
It allows us to see the individuals scores within a set of data as well as the general shape and distribution of the scores.
What is a density plot?
Like a histogram but with a smooth line instead of blocks. The y axis is called density and not frequency.
What does the area under the curve in a density plot always add up to?
1
