Data science up for first round of review

1) Focus on rspec tests 
2) Stuck at trying to refactor the exception part
Instead of passing in arrays as the object to test again, you should look into the the `let(:symbol)` method of rspec, to allow you to pass in an instance of your class, this would removed the need for the helpers class e.g.:
<pre>
describe ChiSquareCalc, 'positive tests' do
...
let(:calculator){StatCalculator.new(....)}

  it "should calculate chi square accurately" do
    expect(calculator.conversion_rate).to eq(your_value)
  end
</pre>