Grouped data

Here data are grouped into “classes”, which are often equal-sized intervals. Instead of seeing the original values, you would be given frequencies or number of values described by a class.

Contents

• Classes (intervals)
• Frequency tables and histograms
  • mean ($\mu$)
  • HL: variance ($\sigma^2$) and standard deviation ($\sigma$) by hand
• Cumulative frequency diagrams
• Calculator

Classes (intervals)

Classes on tests will be equally spaced. For calculations, each class will be represented by its median.

For continuous data, the median is the mean between the start of this class and the start of next class.

For discrete data, the median, is the mean between the start of this class and the end of this class.

Consider the intervals

For continuous data, the mid-interval values are $25$, $35$, and $45$.

For discrete data, say only integers, the mid-interval values are $\frac{21 + 30}{2} = 25.5$, $35.5$, and $45.5$.

For another set of interval classes,

For continuous data, the mid-interval values are $20$, $30$, and $40$.

For whole number data, the mid-interval values are $\frac{15 + 24}{2} = 19.5$, $29.5$ and $39.5$.

Frequency tables and histograms

The mean is estimated using the mid-interval values and frequencies. This is usually done on the calculator.

The modal class is the one with the highest frequency.

The formulas are estimates.

mean ($\mu$)

where $f_x$ is the frequency for the corresponding min-interval values $x$, and

HL: variance ($\sigma^2$) and standard deviation ($\sigma$) by hand

The formula for variance, with frequencies $f$, is

Note that the $- {\bar x}^2$ is outside the summation. Taking the positive square root of both sides is the formula for standard deviation.

Cumulative frequency diagrams

In cumulative frequency diagrams, the $y$-value is the sum of frequencies up to each $x$-value.

When constructing cfd from frequency tables or histograms, At the upper bound of each class, the frequency is added to the existing total (cumulative) frequency. Connect the dots using straight lines.

For quartile (and median) calculations, first find the total number of frequencies (data points). This may or may not be given in the question.

From $x$ values of $25\%$, $50\%$, $75\%$ times the total, read off the $y$ values of $Q1$, $Q2$(median), and $Q3$.

For mean, the diagram should be subdivided into equal classes and enter class mid-interval values and frequencies into calculator. The question should suggest a class size.

Calculator

Group data are still one-variable statistics. Calculator should provide an option to enter frequencies.

Be able to calculate all the statistics as you would for a data list. Interpreting statistics on TI-84 Plus (example using discrete random variables), where frequencies are analogous to probabilities.