HCA contradiction questions

[klavigne]

I just ran some numbers through the HCA. First with the percentages and then with the angles. Everything else stayed the same. But I got two completely different numbers, with the percentages it scored better by a factor of two. Why is this? I''m hoping Garry finds this and can give me the jist of the discrepancies. Which is the more accurate number?

 

[aljdewey]

Angles are more accurate because the data isn''t rounded (up or down).

 

[klavigne]

All numbers are rounded to some extent. If they weren''t the AGS report would say that the Pav angle was 39.8775765426528332462874 and not 39.8, right? Sorry, thats the engineer in me coming out.
Both the numbers I have are rounded to one decimal place.

 

[pricescope]

KPL,

This question has been discussed quite a few times in the past

Here is a short explanation: http://www.pricescope.com/tutor/angl_vs_dpt.asp

It is best to input crown and pavilion angles into HCA. We use trigonometry to calculate the angles from % data. Use angles if you have them because:
1. %''s are often rounded i.e. 13.3% crown height becomes 13% or 13.5%.
2. Pavilion depth % when converted to pavilion angle often understates the pavilion angle by 0.15°
3. Scanners are not good at measuring culets; a culet reduces the pavilion depth % but does not change the pavilion angle.
4. Accuracy of Crown and Pavilion depth measurements depends also on how the Girdle thickness is measured.

Bottom line: if you are getting different HCA for angles and depth %''s, it signals that there are depth measurement errors. Try to add up your crown, girdle and pavilion depths and see whether the sum matches the total depth.

 

[Mara]

This is asked quite often. Percentages are rounded and so therefore inaccurate. If all you have is %'s then use them to get a vague idea of how the stone MIGHT score. aka if it's a 6 on the HCA with % then you know to pass on it and not even request more info. But if it's a 1.5 with %'s then with angles the stone MAY have a chance of being a good performer and you may want to get more info aka angles.

But honestly in my experience with %'s vs angles, if you put %'s in and it gives you a score over 2, I don't think I have ever seen one stone that scored better than that with angles. The %'s are misleading, because often people come here with a 1.5 scoring with % and then are upset when with angles it scores a 3 or so.

ETA: haha i see that everyone responded to this Q at once!

 

[klavigne]

PS and Mara,
Thank-you both for setting my mind at ease. I thought that the % were a rough estimation based on the actual angles.
Now I can have my dessert right, lemon pie!!!!