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MIR Tutorial Bath - Phasing statistics appendix |

Most useful indicators of the validity of a heavy-atom site are the **real
occupancy** and the **atomic thermal parameter(s)**.

Reduction in *R*_{Cullis} when new site is added is also a good indicator.

** R_{Cullis}** =
<

**But** for d_{min} >
» 3.5Å
occupancies and thermal parameters are highly correlated
and the latter should probably not be refined.

To judge a derivative as a whole, check *R*_{Cullis} and the phasing power.

*R*_{Cullis} <
» 0.6 are
considered excellent, <
» 0.9 usable.

For anomalous data any *R*_{Cullis} is useful.

**Phasing power** measures the sharpness of the phase distribution (contribution
of derivative to phasing information compared with noise).

Phasing power = <Heavy-atom amplitude> / <*P*-weighted lack of closure>

> » 1.5 excellent, > » 1.0 good, > » 0.5 usable.

**Note** that all **root mean-square** (RMS) averages are very sensitive to outliers:
underestimates *R*_{Cullis} and phasing power, so straight means are safer.

Check mean & standard deviation of absolute phase difference <|Df|>
between **F**_{P} & **F**_{H}
(**N.B. not applicable in SIR[AS] cases**).

If data accurately scaled, phase of **F**_{P} is uncorrelated with phase
of **F**_{H}.

<|Df|> should be » 90° (±10°).

s should be » 52° for acentric, » 90° for centric.

**Mean figure of merit** measures precision of the "best" phase.

<FOM> » mean of cosine(estimated phase error).