In the dedication Luke explains his method of research

Luk 1:1-4

1 As a number of attempts have been made to put together in order an account of those events which took place among us,
2 As they were handed down to us by those who saw them from the first and were preachers of the word,
3 It seemed good to me, having made observation, with great care, of the direction of events in their order, to put the facts in writing for you, most noble Theophilus;
4 So that you might have certain knowledge of those things about which you were given teaching.