## Summary

```
deriv(<expression>)
```

Returns the per-second rate of change for each time series described by the expression. The results can include both positive and negative rates of change. Use `rate()`

if you want to see only positive rates of change.

## Parameters

Parameter | Description |
---|---|

expression | Expression describing the time series to return rates of change for. |

## Description

The `deriv()`

standard time function returns the rate of change (per second) between adjacent data values in the time series described by the expression. `deriv()`

returns a separate series of results for each time series described by the expression.

You can apply `deriv()`

to any kind of metric. If you are primarily interested in the rates of change of counter metrics, however, you will get more useful results from `rate()`

, which ignores negative rates of change.

### Rate of Change

`deriv()`

finds the per-second rate of change between pairs of adjacent (successively reported) data values.
The rate of change between a pair of data values is computed as follows:

- Subtract the earlier data value from the later value.
- Divide the difference by the number of seconds in the reporting interval.

For example, let’s say that a metric has a reporting interval of 30 seconds, and reports successive data values:

Value | Time |

105,500 | 05:45:00pm |

105,750 | 05:45:30pm |

The `deriv()`

function computes the rate of change between these data values using the following formula: `(105,750 - 105,500)/30`

. The resulting positive value (8.333) is returned at 05:45:30pm, indicating that the metric increased by 8.333 per second between the two values.

Notice that the per-second rate of change between the same two data values would be quite different if the metric had reported them two minutes (120 seconds) apart: `(105,750 - 105,500)/120`

. In this case, the returned per-second rate would be 2.083.

Now let’s say the above metric reports a later value that is less than the earlier value:

Value | Time |

105,750 | 05:45:30pm |

105,250 | 05:46:00pm |

The `deriv()`

function computes the rate of change between these data values using the following formula: `(105,250 - 105,750)/30`

. The resulting negative value (-16.666) is returned at 05:46:00pm, indicating that the metric decreased by 16.666 per second between the two values. (In contrast, `rate()`

simply produces a gap in this case, rather than reporting a negative rate of change.)

## Examples

Here’s a query that shows a sample metric that increments a counter. The reporting interval is 2 seconds, which means the counter increments every 2 seconds. We see the count climb from 3:46:00 to 3:46:16, when it resets to 0. The counter restarts at 3:46:20.

Now we apply `deriv()`

to our original query to find out how fast the counter grows per second. Notice:

`deriv()`

starts reporting at the counter’s second value.- At 3:46:10, the rate of growth is .500, indicating the total has grown half a unit per second since the preceding value.
- A negative rate is reported when the counter falls to 0.