The reproducibility oflichen measurements is poor. Errors may arisefrom mismeasurement ofindividual lichen thalli or from failing to locate large thalli. Observers obtained much more consistent results when a quadrat search was repeated. The reproducibility ofRhizocarpon section Rhizocarpon species measurements increases with increasing sample size but the reverse occursfor Rhizocarpon alpicola measurements. Variations in the extent of error occur between different quadrats, but these appear to be unrelated to the nature of the quadrat terrain. Reproducibility decreases with increasing quadrat size, but the rate ofdecrease slows with larger areas. Individuals vary significantly in their search efficiency. When the errors are calibrated to the data available from Storbreen, southwest Norway, they represent 19-37years on a moraine dated to about 1750 AD