One of the most difficult things to do in observational science is to separate the observer from the observed. For example, in CCD astronomy, we apply bias, dark, and flat-field corrections as well as utilize median combines of shifted images to yield an image that is, ideally, free of any CCD chip defects including differences in pixel sensitivity and zero-point.
We as observers are constrained by other limitations. For example, when we look at a particular galaxy, we observe it from a single vantage point in space and time, a vantage point we cannot change due to our great distance from the object and our existence within an exceedingly short interval of time.
Yet another limitation is a phenomenon that astronomers often call “observational selection”. Put simply, we are most likely to see what is easiest to see. For example, many of the exoplanets we have discovered thus far are “hot Jupiters”. Is this because massive planets that orbit very close to a star are common? Not necessarily. The radial velocity technique we use to detect many exoplanets is biased towards finding massive planets with short-period orbits because such planets cause the biggest radial velocity fluctuations in their parent star over the shortest period of time. Planets like the Earth with its relatively small mass and long orbital period (1 year) are much more difficult to detect using the radial velocity technique. The same holds true for the transit method. Planets orbiting close to a star will transit more often—and are more likely to transit—than comparable planets further out. Larger planets will exhibit a larger Δm than smaller planets, regardless of their location. It may be that Earthlike planets are much more prevalent than hot Jupiters, but we can’t really conclude that looking at the data collected so far (though Kepler has helped recently to make a stronger case for abundant terrestrial planets).
Here’s another important observational selection effect to consider in astronomy: the farther away a celestial object is the brighter that object must be for us to even see it. In other words, many far-away objects cannot be observed because they are too dim. This means that when we look at a given volume of space, intrinsically bright objects are over-represented. The average luminosity of objects seems to increase with increasing distance. This is called the Malmquist bias, named after the Swedish astronomer Gunnar Malmquist (1893-1982).