# Alternate hypothesis being “less,” “greater,” or “two-sided”

Often times, in python's scipy library's statistical tests(e.g. Fisher Exact Test), there is an option for two-tailed or single tailed. But for single-tailed it says "less" or "greater." I don't quite understand what it means when it states:

Use a one-sided alternative hypothesis (also known as a directional hypothesis) to determine whether the population parameter differs from the hypothesized value in a specific direction. You can specify the direction to be either greater than or less than the hypothesized value. Source

Can someone please clarify this? What do they mean by direction? If I have a one tailed distribution with the mean close to 0 and no negative values, then what exactly would "less" do and "greater" do?

Here's an example of my frequency dist.

The x-axis is # of samples and y-axis is number of components of interest and each sample is an image.

• Mean zero and no negative values would imply your distribution has all mass at 0. – Michael M Oct 12 '17 at 20:34
• I've edited the OP. It's close to 0. In other words, my data might fifteen 0s and four or five values between 1-3. @MichaelM – Christian Oct 12 '17 at 20:51

Fisher Exact Tests consider all possible permutations of $2 \times 2$ contingency tables. Each table permutation can be ranked in terms of its degree of association by considering the odds ratio. An odds ratio which is greater than 1 shows a positive association between an exposure and an outcome whereas an OR less than 1 shows a negative association.
$$\begin{array}{c|cc|c} & D & \bar{D} & \text{Total} \\ \hline E & a & b& a+b\\ \bar{E} & c& d& c+d\\ \text{Total} & a+c& b+d& n=a+b+c+d\\ \end{array} \qquad OR = (ad)/(bc)$$