Studies on the initial growth or nucleation of materials and research on selective deposition often mention an incubation time. Many techniques exist to determine the incubation time. The outcome can be very different for each technique when the same nucleation process is considered. For the first time we have given a simple model which shows that several incubation times can be expected if different methods are used. One of the most popular methods, plotting the mass or thickness as a function of time and defining the incubation time as the intercept on the x-axis, is not a good method. In particular, a meaningful incubation time is found only if a layer-by-layer growth mechanism occurs right from the start. Ellipsometry can be used in situ and is a much more sensitive method, but this technique needs more research to correlate the nucleation process with the data obtained using this technique. The determination of the nucleus density using scanning electron microscopy or atomic force microscope is the most accurate method, yet needs a lot of experiments. Without a detailed description of the measurement method the incubation time is a meaningless quantity.