Analytical Methods:Reports
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[edit] Reporting of results
[edit] Significant figures
It is inevitable that there is some uncertainty associated with any measurement whether determined in the field or laboratory. This uncertainty needs to be reflected in the way by which a result is presented. A pH measurement reported as pH 5 implies an actual value in the range of 4.5 to 5.5 whilst pH 5.6 indicates the range 5.55 to 5.65. A decision on this requires some evaluation of the error associated with the determination. A common approach for reporting pH results is to give them to 2 significant figures as in the latter example and the general advice as given by Webster (2001) for reporting soil or sediment analytical results is to present three significant figures, for example
loss on ignition 94.5 g kg-1 phosphorus 5.25 g kg-1
Note that in a number such as 0.001 there is only one significant figure. However, it is undesirable to write for area 12,000 m2; instead this should be stated as 1.2 x 103m2. In this case there are 2 significant figures. Similarly, results on standard deviations or standard errors should be limited to 3 significant figures. Standard deviation is described in any statistical text and expresses variation in sample or measurement values whilst standard errors are the errors associated with means. Standard errors should be given for results derived from replicated measurements and can also be shown as error bars on graphs. As a general rule preference should be given to reporting standard errors of means if there have been adequate measures of the same property.
Webster, R. (2001). Statistics to support soil research and their presentation. European Journal of Soil Science, 52, 331 – 340.
[edit] Units
The need for a standard internationally accepted set of units and abbreviations is obvious and extensive use is made of the ‘le Système International d’Unités,’ usually referred to as the SI system. The core of the system is seven base units, but only three are of widespread applicability to geoarchaeology, namely length (metre, symbol m), mass (kilogram, symbol kg) and time (second, symbol s). In practice, it is common to encounter t instead of Mg for tonnes, cm for centimetre, ha for hectare and yr for year.
A set of prefixes as given below is widely used to cope with very large or very small quantities.
| Multiplication factor | Prefix | Symbol |
|---|---|---|
| 1012 | tera | T |
| 109 | giga | G |
| 106 | mega | M |
| 103 | kilo | k |
| 10-1 | deci | d |
| 10-2 | centi | c |
| 10-3 | milli | m |
| 10-6 | micro | μ |
| 10-9 | nano | n |
| 10-12 | pico | p |
So very obviously 12,000m can be written as 12km, 0.000034 g as 34 μg or 34 x 10-6 g. Note that symbols should not change in the plural and should not be followed by a full stop except at the end of a sentence.
