Standard Deviation

StdDev is an aggregate function that calculates the standard deviation of a specified property. It requires these other measures to be defined on the property:

• Sum - to calculate the sum of the property values.
• Count - to count the number of values.
• SumSquare - to calculate the sum of the squares of the property values.

StdDev is a virtual measure and does not store its value in the database.

Example

use Doctrine\ORM\Mapping as ORM;
use Rekalogika\Analytics\Core\AggregateFunction\Count;
use Rekalogika\Analytics\Core\AggregateFunction\StdDev;
use Rekalogika\Analytics\Core\AggregateFunction\Sum;
use Rekalogika\Analytics\Core\AggregateFunction\SumSquare;
use Rekalogika\Analytics\Core\Metadata as Analytics;

class OrderSummary extends Summary
{
    #[ORM\Column(type: Types::INTEGER)]
    #[Analytics\Measure(
        function: new Sum('item.price'),
    )]
    private ?int $price = null;

    #[ORM\Column(type: Types::FLOAT)]
    #[Analytics\Measure(
        function: new SumSquare('item.price'),
        hidden: true,
    )]
    private ?float $priceSumSquare = null;

    #[ORM\Column(type: Types::INTEGER)]
    #[Analytics\Measure(
        function: new Count('id'),
    )]
    private ?int $count = null;

    #[Analytics\Measure(
        function: new StdDev(
            sumSquareProperty: 'priceSumSquare',
            countProperty: 'count',
            sumProperty: 'price',
        ),
    )]
    private ?int $priceStdDev = null;
}

note

It is not necessary to store the standard deviation value in the database, and therefore the property does not need the ORM\Column attribute.

SumSquare Function

The SumSquare function is used to calculate the sum of the squares of the specified property values. It is a building block for many statistical functions, including StdDev. However, the end users are probably not interested in the result of the SumSquare function itself, and you can hide it from the UI by using the hidden parameter in the Measure attribute like in the example above.