Baseball statistics play an important role in evaluating a player's and/or team's progress. 

Since the flow of a baseball game has natural breaks to it, and normally players act individually rather than performing in clusters, the sport lends itself to easy record-keeping and statistics. Statistics have been kept for professional baseball since the creation of the National League and American League, now part of Major League Baseball. 

Many statistics are also available from outside of Major League Baseball, from leagues such as the National Association of Professional Base Ball Players and the Negro Leagues, although the consistency of whether these records were kept, of the standards with respect to which they were calculated, and of their accuracy varied. 

The practice of keeping records of player achievements was started in the 19th century by Henry Chadwick. Based on his experience with the sport of cricket, Chadwick devised the predecessors to modern-day statistics including batting average, runs scored, and runs allowed. 

Traditionally, statistics such as batting average (the number of hits divided by the number of at bats) and earned run average (the average number of earned runs allowed by a pitcher per nine innings) have dominated attention in the statistical world of baseball. However, the recent advent of sabermetrics has created statistics drawing from a greater breadth of player performance measures and playing field variables. Sabermetrics and comparative statistics attempt to provide an improved measure of a player's performance and contributions to his team from year to year, frequently against a statistical performance average.
When did the record keeping for players first start?
By who?
What sport was he familiar with?
Did he keep track of batting averages and runs scored?
What else?
What is the number of hits divided by the number of at bats?
And the average number of earned runs allowed by who per nine innings is the earned run average?
Are those stats the most popular?
Of which sport?
What's the new way to get statistics from more player measures?
Is that a more accurate way to judge a player's abilities?
Does baseball have built in breaks to it?
Are the players individually acting or in groups?
Does that make it harder or easier for record keeping?
Can you get records of the Negro Leagues?
Are they completely accurate?
Was the way they were calculated a problem?
Name one league that is now part of the Major League.
And another?
in the 19th century
 in the 19th century 
Henry Chadwick
Henry Chadwick
cricket
cricket
yes
batting average, runs scored,
runs allowed
runs allowed
batting average
batting average
yes
earned run average (the average number of earned runs allowed by a pitcher per nine innings)
yes
have dominated attention
baseball
 baseball
sabermetrics
sabermetrics
yes
Sabermetrics and comparative statistics attempt to provide an improved measure
yes
natural breaks to it
individually
 and normally players act individually
easier
easy record-keeping
yes
and the Negro Leagues
no
accuracy varied
yes
to which they were calculated
American League
American League
the National League
the National League