Observations of variable quasar candidates

A sample of quasar candidates selected on the basis of variability has been investigated with slit spectroscopy. The results show that this technique is extremely powerful in terms of success rate, and is not affected by strong biases up to at least z ≅ 2.6. The population of quasars found appears to be the same as that picked out with the slitless spectroscopy or the multi-colour techniques. The present method is therefore well suited for the construction of statistically significant quasar samples, allowing, when used in conjunction with complementary approaches, a reliable assessment of their incompleteness. (author)