`foldchange`

computes the fold change for two sets of values.
`logratio2foldchange`

converts values from log-ratios to fold
changes. `foldchange2logratio`

does the reverse.

1 2 3 | ```
foldchange(num,denom)
logratio2foldchange(logratio, base=2)
foldchange2logratio(foldchange, base=2)
``` |

`num,denom` |
vector/matrix of numeric values |

`logratio` |
vector/matrix of log-ratio values |

`foldchange` |
vector/matrix of fold-change values |

`base` |
Exponential base for the log-ratio. |

Fold changes are commonly used in the biological sciences as a
mechanism for comparing the relative size of two measurements. They
are computed as: *num/denom* if
*num>denom*, and as *-denom/num*
otherwise.

Fold-changes have the advantage of ease of interpretation and symmetry
about *num=denom*, but suffer from a discontinuty between -1 and
1, which can cause significant problems when performing data
analysis. Consequently statisticians prefer to use log-ratios.

A vector or matrix of the same dimensions as the input containing the converted values.

Gregory R. Warnes greg@warnes.net

1 2 3 4 5 6 | ```
a <- 1:21
b <- 21:1
f <- foldchange(a,b)
cbind(a,b,f)
``` |

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