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3. Molecular Dimensions

It is important that some basic principles of conformation be taught to students. They are added here for completeness.

Accuracy is a measure of the deviation of an observed quantity from its correct value. Since we never know the truly correct value we don't know how accurate our data are.

Precision is a measure of the experimental uncertainty in a measured quantity, an indication of its reproducibility. This is what we measure and is what we indicate in an e.s.d.

Precision of Measurements . It is important to stress that a value quoted as 1.395(4) Å means 1.395 $\pm$ 0.0004 Å where $\pm$ means an estimated standard deviation (e.s.d.) (derived from the inverted least squares matrix used in refining the structure). If the distribution of errors is normal, then there is a 99% chance that a given measurement will differ by less than 2.7 times the e.s.d. from the mean. A bond length of 1.542(7) Å (1.542 Å with an e.s.d. of 0.007 Å) is probably not significantly different from one measured at 1.527(7) Å.

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