The degree of cross-matching, that is the measure of similarity between sample and reference, is denoted by a "t-value"; the higher the value the greater the similarity.

The greater the similarity the greater is the probability that the patterns of samples and references have been produced by growing under the same conditions at the same time.

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Apart from general commercial undertakings, the Laboratory has also been involved in several major research projects on buildings, the environment and ecology, and, most recently, climate change.

The Laboratory maintains academic links with a number of University units, is involved with public lecture programmes, and provides advice and training to other dendrochronologists.

Furthermore, combining samples in this way to make a site chronology usually has the effect of increasing the time-span that is under comparison.

As also mentioned above, the longer the period of growth under consideration, the greater the certainty of the cross-match.

The pattern of a short period of growth, 20, 30 or even 40 consecutive years, might conceivably be repeated two or even three times in the last one thousand years.

A short pattern might also be repeated at different time periods in different parts of the country because of differences in regional micro-climates.Tree-ring dating relies on a few simple, but quite fundamental, principles.Firstly, as is commonly known, trees (particularly oak trees, the most frequently used building timber in England) grow by adding one, and only one, growth-ring to their circumference each, and every, year.When samples from the same phase do cross-match with each other they are combined at their matching positions to form what is known as a "site chronology".As with any set of data, this has the effect of reducing the anomalies of any one individual (brought about in the case of tree-rings by some non-climatic influence) and enhances the overall climatic signal.It is less likely, however, that such problems would occur with the pattern of a longer period of growth, that is, anything in excess of 54 years or so.