A dichotomous key

`1. Direct Gradient Analysis................................2`
`  2. Few species...........................................4`
`     4. Monotonic responses to gradients (low beta).......`
`              .............................Linear regression`
`     4. Nonmonotonic responses to gradients.(high beta)......`
`              .....................Generalized linear models`
`  2. Many species..........................................5`
`     5. Monotonic responses  ............................RDA*`
`     5. Nonmonotonic responses.............................6`
`        6. concerned about arch effect..................DCCA`
`        6. not concerned about arch effect...............CCA`
`1. Indirect Gradient Analysis..............................3`
`  3. Only distance values are available....................7`
`     7. Monotonic responses ............................PCoA`
`     7. Nonmonotonic responses..........................NMDS`
`  3. Raw data available....................................8`
`     8. Monotonic responses ...............................9*`
`        9. Variables noncommensurate......PCA - corr. matrix`
`        9. Variables commensurate..........PCA - cov. matrix`
`     8. Nonmonotonic responses............................10`
`        10. Feel OK about prespecifying number of dimensions,`
`            not worried about local optima, not interested in `
`            species scoes..............NMDS`
`        10. Not as above, but willing to accept either  arch`
`            effect or detrending/rescaling................11`
`            11. Don't like arch, detrending OK ..........DCA`
`            11. Arch OK, or only interested in axis 1.....CA`

The above is just meant as a starting point for discussion. Clearly, the issues are too complex to reduce the decision about which method to use to a dichotomous key. Suggestions for revision are welcome!

*Recently, ter Braak and  Smilauer (1998) have pointed out that Correspondence Analysis (and by implications, its derivatives DCA, CCA and DCCA) has two 'faces': a unimodal face and a linear face.  This implies that dichotomies #5 and  #8 may be outdated.  It seems that the major difference between PCA/RDA and the 'linear face' of CA/CCA is that the latter is better for species composition, and not for overall trends in abundance of all species combined.