Re: Variability statistics

[Robert Creager <Robert_Creager@LogicalChaos.org>]

Hey Michael,

Just as a point of comparison, on a small subset of my data (13 nights)
with 880414 pairs of measurements, I calculate:

tassiv_6=# select max(index), min(index), avg(index), stddev(index) from
welch_index ;
   max   |   min    |        avg        |      stddev      
---------+----------+-------------------+------------------
 266.943 | -58.6898 | 0.837757885348059 | 3.94842816122941


I'm only calculating the index for stars with 10 or more measurements, using the
formula:

avg_i = sum(mag_i / (sigma_mag_i^2 + 0.01^2)) / sum(1/(sigma_mag_i^2 + 0.01^2))
avg_v = sum(mag_v / (sigma_mag_v^2 + 0.01^2)) / sum(1/(sigma_mag_v^2 + 0.01^2))

index = sum(  (mag_i - avg_i) / sqrt( sigma_mag_i^2 + 0.01^2)
            * (mag_v - avg_v) / sqrt( sigma_mag_v^2 + 0.01^2) )
        / sqrt( pair_count * (pair_count - 1) )

Where sigma_mag is the error term returned via Daophot.  I've no recollection of
where I picked up the calculation I'm using, although I believe it's from one of
the scripts that was floating around on this list a few months ago.

Keep in mind that I'm still working on the data reduction, currently playing
with having color terms fixed for multiple nights of data, and seeing
what that actually does to my data.

Cheers,
Rob

When grilled further on (Tue, 13 Jan 2004 19:34:45 -0600),
Michael Sallman <msallman@pro-ns.net> confessed:

> mysql> select max(ws),min(ws),avg(ws),stddev(ws) from star_stats;
> +----------+---------+----------+------------+
> | max(ws)  | min(ws) | avg(ws)  | stddev(ws) |
> +----------+---------+----------+------------+
> | 17143.49 |    NULL | 6.412805 |  36.235592 |
> +----------+---------+----------+------------+
> 1 row in set (13.28 sec)
> 
> mysql> select max(ws),min(ws),avg(ws),stddev(ws) from test_star_stats;
> +---------+----------+----------+------------+
> | max(ws) | min(ws)  | avg(ws)  | stddev(ws) |
> +---------+----------+----------+------------+
> | 4901.17 | -1614.17 | 1.088818 |  12.909869 |
> +---------+----------+----------+------------+
> 1 row in set (22.15 sec)
> 
> mysql> select max(ws),min(ws),avg(ws),stddev(ws) from stetson_stats;
> +---------+---------+----------+------------+
> | max(ws) | min(ws) | avg(ws)  | stddev(ws) |
> +---------+---------+----------+------------+
> |   80.56 |  -25.69 | 0.687546 |   1.358058 |
> +---------+---------+----------+------------+
> 1 row in set (3.91 sec)
> 

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