In order to calculate the biggest increase over the previous year, let's calculate the difference between each pair of consecutive years:
![\begin{gathered} 2001\text{ to 2002:} \\ 6.5-5.5=1 \\ 2002\text{ to 2003:} \\ 7-6.5=0.5 \\ 2003\text{ to 2004:} \\ \text{7}.75-7=0.75 \\ 2004\text{ to 2005:} \\ 8.5-7.75=0.75 \\ 2005\text{ to 2006:} \\ 10.5-8.5=2 \\ 2006\text{ to 2007:} \\ 11.5-10.5=1 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/6ggq338dgh65vhi3pl0lciipwwcr3efoij.png)
We can see that the biggest increase over the previous year occurred in 2006 (an increase of $2 in relation to 2005).