So, do you think the sample was biased? If so, you are right. When you look at the brightest stars in the night sky, you tend to see the largest, hottest, and most luminous stars. Of the 26 brightest stars, nearly all are much larger and brighter than average.
Your goal should be to make a complete H-R diagram – one that will show all the major types of stars, and roughly how many stars of each type the universe contains. Making an H-R diagram with the brightest stars didn’t work, because the diagram contained too many bright stars. You need to try another approach.
Next, you should make an H-R diagram using the nearest stars rather than the brightest. This diagram will tell us what the stars closest to the Sun are like.