A few students have asked me about how to use 'double', 'twice as', 'three times', 'twofold', 'threefold' etc. Compare how each word/phrase is used in the following examples:
1. 'double' (verb)
The number of unemployed people doubled between 2005 and 2009.
2. 'twice as...as/compared to', 'three times as...as/compared to'
There were twice as many unemployed people in 2009 as in 2005.
Twice as many people were unemployed in 2009 compared to 2005.
3. 'twofold', 'threefold' (adjective or adverb)
There was a twofold increase in the number of unemployed people between 2005 and 2009. (adjective with the noun 'increase')
The number of unemployed people increased twofold between 2005 and 2009. (adverb with the verb 'increase')
Try using these forms in your own sentences. Make sure you follow the patterns.