Dear Mahsa Nemati please select the "precipitation" field as it provides Surface Precipitation Estimate for the area of interest. While, the precipitation_cnt layer provides information about Surface Convective Rain Rate and "randomError" provides the Random Error Estimate of the layer. The IRprecipitation field provides the rain destination in IR wavelength.

Please follow the user guides for TRMM using below links:

1)https://disc2.gesdisc.eosdis.nasa.gov/data/TRMM_L3/TRMM_3B42/doc/README.TRMM_V7.pdf

2) https://disc.gsfc.nasa.gov/datasets/TRMM_3B42_Daily_7/summary

Mahsa Nemati I agree with the Gomal Amin you need to select the first variable. However i think the cnt here is the count of valid retrievals in a grid cell. Also, it depends on the study area you are working with and the type of analysis you are doing. The The High Quality ""HQPrecipitation" is a combined product from microwave precipitation estimate averaged by combination of all currently available TCI, TMI, SSMI, SSMIS, AMSR-E, AMSU-B, and MHS estimates. I suggest you to read the document below before you set your goal.

https://gpm.nasa.gov/sites/default/files/document_files/3B42_3B43_doc_V7.pdf

Check the attached table:

(Ref.: README Document for the Tropical Rainfall Measurement Mission (TRMM) Version 7, Page#14 )

So, if you're working on surface precipitation rate (mm hr-1), use the 1st field.