"Quantitative research is a systematic process used to gather and statistically analyze information that has been measured by an instrument. Instruments are used to convert information into numbers. It studies only quantifiable concepts (concepts that can be measured and turned into numbers)." It examines phenomenon through the numerical representation of observations and statistical analysis.

Langford, R. ( 2000). Navigating the Maze of Nursing Research. Elsevier.

If you want to limit your search to quantitative  studies, first try "quantitative" as a keyword, then try using one of the following terms/phrases in your search (example: lactation AND statistics):

Correlational design*

Effect size

Empirical research

Experiment*

Hypothesis

Quasi-experiment*

Reliability

Sampl*

Statistics

Validity

Variables