# SEM

Running standard error of the mean using Welford's algorithm.

## Parameters

- **ddof**

  *Type* → `int`

  *Default* → `1`

  Delta Degrees of Freedom. The divisor used in calculations is `n - ddof`, where `n` is the number of seen elements.

## Attributes

- **n** (`int`)

  Number of observations.

## Examples

```python
from river import stats

X = [3, 5, 4, 7, 10, 12]

sem = stats.SEM()
for x in X:
    sem.update(x)
    print(sem.get())
```

```text
0.0
1.0
0.577350
0.853912
1.240967
1.447219
```

```python
from river import utils

X = [1, 4, 2, -4, -8, 0]

rolling_sem = utils.Rolling(stats.SEM, window_size=3, ddof=1)
for x in X:
    rolling_sem.update(x)
    print(rolling_sem.get())
```

```text
0.0
1.5
0.881917
2.403700
2.905932
2.309401
```

## Methods

> [!NOTE]
>
> **get**
>
> Return the current value of the statistic.

> [!NOTE]
>
> **revert**

> [!NOTE]
>
> **update**
>
> Update the called instance.
>
> **Parameters**
>
> - **x** — `float`
> - **w** — `float` — defaults to `1.0`

> [!NOTE]
>
> **update\_many**

## References

- [Wikipedia article on algorithms for calculating variance](<https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Covariance>)