Receiver Operating Curve (ROC)

#1
Hi All!

i am stuck on ROC analysis.I would be too pleased if you let me know how to plot ROC curve for below clinical data.

First column shows numeric values for a quaintly called FFR and second column shows the numeric values for another quantity called DS. I know that FFR value below 0.75 is considered to be abnormal and above 0.75 to be normal. Considering this issue, I want to find optimal cut-off value for DS to predict an abnormal FFR (<0.75). Please let me know how I can do that.

FFR DS
0.53 20
0.37 61
0.36 68
0.7 51
0.44 74.5
0.67 59.5
0.86 53.5
0.65 62.5
0.32 75.5
0.49 67.5
0.62 60
0.43 61
0.42 75.5
0.72 68.5
0.63 58
0.37 57
0.61 64.5
0.71 44
0.59 60
0.65 70
0.22 73
0.77 47.5
0.84 56.5
0.69 50.5
0.69 52
0.75 50
0.50 50.5
0.65 58
0.65 39
0.75 51
0.66 56.5
0.5 61.5
0.31 57
0.46 62.5
0.68 40
0.67 53
0.79 54.5
0.35 47
0.87 30
0.46 67.5
0.89 21
0.83 20
0.30 66.5
0.75 56.5
0.59 51
0.76 47
0.49 45
0.75 52.5
0.78 41
0.76 54.5
0.84 37.5
0.91 42
0.61 58
0.80 44.5
0.84 36
0.89 53
0.95 61.5

Thank you very much
Ashton