Because density of balsa wood is so important we now offer three grades of Balsa:
1) Medium Density Balsa unselected: 10-19 pounds/cu.ft
Range of densities expected in any sufficiently large random sample is 10-19 pcf, with usual expected outliers. Distribution is a fairly broad normal curve with average near center of range. Piece-weight is calculated by dividing sheet weight by number of sticks/sheet. Not individually weighed. 

2) High Density: mean density 21 pounds/cu.ft, nominal range 19 - 24 pcf
Range of densities expected in any sufficiently large random sample is 19-28 pcf. Distribution is a tight normal curve (small variance) sharply skewed to low end of range, with average = 21 pcf. Pieces are individually weighed.  Higher weight and strength, greater stiffness. 

3) Low Density: mean density 8 pcf;  nominal range 6-10 pounds/cu.ft
Range of densities expected in any sufficiently large random sample is 6-10 pcf. Distribution is a tight normal curve (small variance) with average = approximately 8 pcf. 
Pieces are individually weighed.  Light weight, less stiffness.

Insider Tip: Balsa varies considerably in its density, for optimum results, each piece should be weighed to determine its suitability for use in a particular part of your structure.