# Pandas DataFrame: ewm() function

## DataFrame - ewm() function

The ewm() function is used to provide exponential weighted functions.

**Syntax:**

DataFrame.ewm(self, com=None, span=None, halflife=None, alpha=None, min_periods=0, adjust=True, ignore_na=False, axis=0)

**Parameters:**

Name | Description | Type/Default Value | Required / Optional |
---|---|---|---|

com | Specify decay in terms of center of mass, α=1/(1+com), for com≥0. |
float | Optional |

span | Specify decay in terms of center of mass, α=1/(1+com), for com≥0. |
float | Optional |

halflife | Specify decay in terms of half-life, α=1−exp(log(0.5)/halflife),forhalflife >0. |
float | Optional |

alpha | Specify smoothing factor α directly, 0<α≤1. | float | Optional |

min_periods | Minimum number of observations in window required to have a value (otherwise result is NA). | int Default Value: 0 |
Required |

adjust | Divide by decaying adjustment factor in beginning periods to account for imbalance in relative weightings (viewing EWMA as a moving average). | bool Default Value: True |
Required |

ignore_na | Ignore missing values when calculating weights; specify True to reproduce pre-0.15.0 behavior. | bool Default Value: False |
Required |

axis | The axis to use. The value 0 identifies the rows, and 1 identifies the columns. | {0 or 'index', 1 or 'columns'} Default Value: 0 |
Required |

**Returns:** DataFrame

A Window sub-classed for the particular operation.

**Notes:**

Exactly one of center of mass, span, half-life, and alpha must be provided. Allowed values and relationship between the parameters are specified in the parameter descriptions above; see the link at the end of this section for a detailed explanation.

When adjust is True (default), weighted averages are calculated using weights (1-alpha)**(n-1), (1-alpha)**(n-2), …, 1-alpha, 1.

When adjust is False, weighted averages are calculated recursively as:

weighted_average[0] = arg[0]; weighted_average[i] = (1-alpha)*weighted_average[i-1] + alpha*arg[i].

When ignore_na is False (default), weights are based on absolute positions. For example, the weights of x and y used in calculating the final weighted average of [x, None, y] are (1-alpha)**2 and 1 (if adjust is True), and (1-alpha)**2 and alpha (if adjust is False).

When ignore_na is True (reproducing pre-0.15.0 behavior), weights are based on relative positions. For example, the weights of x and y used in calculating the final weighted average of [x, None, y] are 1-alpha and 1 (if adjust is True), and 1-alpha and alpha (if adjust is False).

**Example: **

Download the Pandas DataFrame Notebooks from here.

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