Fig. 3.
The correlation between the extraction time and the difficulty index. The correlation coefficient was 0.584, suggesting a moderate positive correlation (P<0.001).
Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.