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Version: Java (Groovy)

varBy

varBy returns the variance for each group. Null values are ignored.

caution

Applying this aggregation to a column where the variance cannot be computed will result in an error. For example, the variance is not defined for a column of string values.

Syntax​

table.varBy()
table.varBy(groupByColumns...)

Parameters​

ParameterTypeDescription
groupByColumnsString...

The column(s) by which to group data.

  • NULL returns the variance for all non-key columns.
  • "X" will output the variance of each group in column X.
  • "X", "Y" will output the variance of each group designated from the X and Y columns.
groupByColumnsColumnName...

The column(s) by which to group data.

  • NULL returns the variance for all non-key columns.
  • "X" will output the variance of each group in column X.
  • "X", "Y" will output the variance of each group designated from the X and Y columns.
groupByColumnsCollection<String>

The column(s) by which to group data.

  • NULL returns the variance for all non-key columns.
  • "X" will output the variance of each group in column X.
  • "X", "Y" will output the variance of each group designated from the X and Y columns.

Returns​

A new table containing the variance for each group.

How to calculate variance​

  1. Find the mean of the data set. Add all data values and divide by the sample size nn.
xˉ=∑i=1nxin\bar{x} = \frac{\sum_{i=1}^{n}{x_i}}{n}
  1. Find the squared difference from the mean for each data value. Subtract the mean from each data value and square the result.
(xi−xˉ)2(x_i - \bar{x})^2
  1. Find the sum of all the squared differences. The sum of squares is all the squared differences added together.
SS=∑i=1n(xi−xˉ)2SS = \sum_{i=1}^{n}{(x_i - \bar{x})^2}
  1. Calculate the variance. Variance is the sum of squares divided by the number of data points. The formula for variance for a sample set of data is:
s2=Σ(xi−xˉ)2n−1s^2 = \frac{\Sigma (x_i - \bar{x})^2 }{n-1}

Examples​

In this example, varBy returns the variance of the whole table. Because the variance cannot be computed for the string columns X and Y, these columns are dropped before applying varBy.

source = newTable(
    stringCol("X", "A", "B", "A", "C", "B", "A", "B", "B", "C"),
    stringCol("Y", "M", "N", "O", "N", "P", "M", "O", "P", "M"),
    intCol("Number", 55, 76, 20, 130, 230, 50, 73, 137, 214),
)

result = source.dropColumns("X", "Y").varBy()

In this example, varBy returns the variance, as grouped by X. Because the variance cannot be computed for the string column Y, this column is dropped before applying varBy.

source = newTable(
    stringCol("X", "A", "B", "A", "C", "B", "A", "B", "B", "C"),
    stringCol("Y", "M", "N", "O", "N", "P", "M", "O", "P", "M"),
    intCol("Number", 55, 76, 20, 130, 230, 50, 73, 137, 214),
)

result = source.dropColumns("Y").varBy("X")

In this example, varBy returns the variance, as grouped by X and Y.

source = newTable(
    stringCol("X", "A", "B", "A", "C", "B", "A", "B", "B", "C"),
    stringCol("Y", "M", "N", "O", "N", "P", "M", "O", "P", "M"),
    intCol("Number", 55, 76, 20, 130, 230, 50, 73, 137, 214),
)

result = source.varBy("X", "Y")