To illustrate Kohlschreiber's consistency in the rankings, let's compare him to Fernando Verdasco. Now, while Verdasco is famous for his inconsistency, his rankings have actually been relatively stable across year-ends: he's been in the top 50 for 15 straight years (every year since 2004), and unlike Kohlschreiber, he is on course for another year in the top 50 in 2019, standing at #34 in the race. Verdasco is also a good point of comparison because he's a near contemporary of Kohlschreiber: Verdasco was born on 11 November 1983, Kohlschreiber on 16 October 1983. Both have been consistent, but Kohlschreiber more so.
Number of years in a row in top 50: Verdasco 15 (starting 2004), Kohlschreiber 12 (starting 2007).
Highest ranking: Verdasco: #7, Kohlschreiber #16.
Highest year-end ranking: Verdasco #9, Kohlschreiber #20.
Lowest year-end ranking since start of Kohlschreiber's streak: Verdasco #49, Kohlschreiber #43.
Number of year-ends in top 20: Verdasco 3, Kohlschreiber 1.
Number of year-ends ranked 21-30: Verdasco 5, Kohlschreiber 5.
Number of year-ends ranked 31-40: Verdasco 5, Kohlschreiber 5. [N.B. Three of Verdasco's finishes in this range were the three before Kohlschreiber broke into the top 50: Verdasco's year-end ranking was 36, 32, and then 35 in 2004, 2005, and 2006, respectively].
Number of year-ends ranked 41-50: Verdasco 2, Kohlschreiber 1.