velout | R Documentation |

Quickly identifies putative outliers in a large number of growth curves.

```
velout(x, y, id, data, lag = 1, velpower = 0.5, limit = 5, linearise = FALSE)
```

`x` |
age vector. |

`y` |
outcome vector, typically weight or height. |

`id` |
factor identifying each subject. |

`data` |
data frame containing x, y and id. |

`lag` |
lag between measurements for defining growth velocity. |

`velpower` |
a value, typically between 0 and 1, defining the power of delta x to use when calculating velocity as delta(y)/delta(x)^velpower. The default of 0.5 is midway between velocity and increment. |

`limit` |
the number of standard deviations beyond which a velocity is deemed to be an outlier. |

`linearise` |
if TRUE y is converted to a residual about the median curve of y versus x. |

The algorithm works by viewing serial measurements in each growth curve as triplets (A-B-C) and comparing the velocities between them. Velocity is calculated as

diff(y, lag = lag) / diff(x, lag = lag) ^ velpower

Missing values for x or y are ignored. If any of the AB, BC or AC velocities
are abnormal (more than `limit`

SDs in absolute value from the median
for the dataset) the code for B is non-zero.

Returns a data frame with columns: id, x, y (from the call), code (as described below), vel1, vel2 and vel3 (corresponding to the velocities AB, BC and AC above). The 'data' attribute contains the name of 'data'.

Code is a factor taking values between 0 and 8, with 0 normal (see table
below). Values 1-6 depend on the pattern of abnormal velocities, while 7 and
8 indicate a duplicate age (7 for the first in an individual and 8 for later
ones). Edge outliers, i.e. first or last for an individual, have just one
velocity. Code 4 indicates a conventional outlier, with both AB and BC
abnormal and AC normal. Code 6 is an edge outlier. Other codes are not
necessarily outliers, e.g. codes 1 or 3 may be adjacent to a code 4. Use
`codeplot`

to look at individual curves, and `zapvelout`

to delete
outliers.

code | AB+BC | AC | interpretation |

0 | 0 | 0 | no outlier |

0 | 0 | NA | no outlier |

1 | 0 | 1 | rare pattern |

2 | 1 | 0 | complicated - look at curve |

3 | 1 | 1 | adjacent to simple outlier |

4 | 2 | 0 | single outlier |

5 | 2 | 1 | double outlier |

6 | 1 | NA | edge outlier |

7 | - | - | first duplicate age |

8 | - | - | later duplicate age |

Tim Cole tim.cole@ucl.ac.uk

`codeplot`

, `zapvelout`

```
outliers <- velout(age, height, id, heights, limit=3)
```

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