For a function of two variables z=f(x,y), the gradient is the two-dimensional vector <f_x(x,y),f_y(x,y)>. This definition generalizes in a natural way to functions of more than three variables. For the function z=f(x,y)=4x^2+y^2.
By this definition, about 10% of people who smoke marijuana become addicted to it. (More on Time.com: PHOTOS â Cannabis Conventions). However, nonspecialists (including many doctors) still tend to use an older perspective, now seen as outdated by experts.
They did find a decline in IQ in frequent smokers who started smoking as adults. Fascinatingly enough, most scientists will tell you that p<0.05 is the significant level, not 0.11. One IOH study of marijuana use did find with p<0.05 that users experience lower cancer rates, an actually significant statistic.