Answer:
Well, I would classify it as a comma splice (i.e., using a comma to "connect" two independent clauses).
Explanation:
Here are the ways to correct the error.
by using a period ....
It is estimated that ten percent of the population has a fear of needles. Medical researchers are developing devices that make injections pain-free.
by using a semi-colon ....
It is estimated that ten percent of the population has a fear of needles; medical researchers are developing devices that make injections pain-free.
by using a coordinating conjunction ....
It is estimated that ten percent of the population has a fear of needles, so medical researchers are developing devices that make injections pain-free.
by using a conjunctive adverb ....
It is estimated that ten percent of the population has a fear of needles; therefore, medical researchers are developing devices that make injections pain-free.
