...and over a three-year period this percentage was ...
1) THREE YEAR OR THIRTY YEAR ? Be careful of this.
... category was lower than THE others and this...
2) ARTICLE "THE"
In 1981, the percentage of'resources e.g books' category was same at 15 percent.
Besides that, in 1991 the percentage of 'resources e.g books' category was greater than 'furniture and equipment' category 20 and ...
In contrasscontrast, in 2001 this category was lower than 'furniture and equipment' category at 9 and ...
3) REPETITION. You should AVOID THIS !
4) TYPO . contrass ---> CONTRAST