The NASDAQ 100 has is up about 284 points year to date, about 7%. However not all stocks in this index are treated equally. This index is 100 of the largest non-financial companies listed on the NASDAQ. It is a modified capitalization-weighted index.
Capitalization-weighted tends to reflect the movement of the largest companies based on “market capitalization”.
Therefore here is how the NASDAQ 100 really performed:
Amazon: 86 points – 30% of the 284 points
Google: 66 points – 23% of the 284 points
Over 50% of the entire NASDAQ rise in 2015 has been in just 2 stocks.