There can be personal bias in reading non-digital instruments or estimating certain quantities. This is referred to as the ‘personal equation’. One example is a mercury thermometer. Mercury has a lower surface tension in contact with glass, then in contact with air, so a meniscus forms on top of the mercury column in a glass tube. This results in a subjective reading, notably in estimating values between the marks on the scale (1). Another example is the estimation of cloudiness. Apparently many people prefer reporting cloudiness as 1 or 3 or 7 oktas rather than other values (2).
Sometimes a discontinuity in a time series of station temperature, cloudiness, or other meteorological variable can be attributed to a staff change. The personal equation is an important factor in other sciences such as anthropometry, demography, geography, and physics (1).
(1) Cox, N.J. 1991. Human factors. Nature 353, 597.
(2) Linacre, E.T. 1992. Climate Data & Resources (Routledge) 366pp