I'm not sure there's much real scientific research on preparation (at least when this was published in the 90s), especially as it can be quite subjective. There's a bit about factors affecting caffeine content on p.425:

> The quantity of caffeine that infuses into a tea brew is determined by infusion time and by leaf style. Longer infusion times lead to greater quantities of caffeine in a tea beverage. Smaller sized tea leaves give a more rapid and stronger infusion, whereas larger leaves and uncut leaves lead to weaker infusions. This results in more or less caffeine extraction, respectively. The caffeine content of a typical tea beverage will range from 20 to 70 mg per 170 ml of infusion, with a typical infusion being prepared from about 2 to 2.5 g of tea leaves. Coffee brews typically contain from 40 to 155 mg caffeine per 170 ml beverage. There has been little research done on the pharmacology of tea-beverage caffeine. One study suggests a dose of caffeine from tea has a different physiological effect than a pure dose of caffeine (Das et al., 1965). This has been attributed to the amino acid theanine, which is unique to tea. However, there are no well-designed clinical studies to support this position. The consensus among scientists today is that caffeine from all beverage sources has a similar physiological effect. The actual content of caffeine depends on many factors, particularly the method of brewing. A brew prepared by the Chinese "gong-fu" style is likely to have a different caffeine impact compared with the Western style of loose tea or to that from a tea bag (Hicks et al., 1996). Some reports have suggested that green tea contains significantly less caffeine than black tea. This may be influenced by the clone of leaf used to produce the tea or by the impact of different brewing techniques. No significant differences have been found when brewing green and black teas under similar conditions (Hicks et al., 1996), discrediting the theory that withering and fermentation have a significant impact on caffeine content (Sanderson, 1972).