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Data Analysis

Data Analysis. Do it yourself!. What to do with your data?. Report it to professionals (e.g., AAVSO) Excellent! A real service to science; don’t neglect this Publish observations (e.g., JAAVSO) Analyze it – yourself!. But …. I’m not a mathematician Let the computer do the math

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Data Analysis

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  1. Data Analysis Do it yourself!

  2. What to do with your data? • Report it to professionals (e.g., AAVSO) • Excellent! A real service to science; don’t neglect this • Publish observations (e.g., JAAVSO) • Analyze it – yourself!

  3. But … • I’m not a mathematician • Let the computer do the math • I’m not a programmer • Get programs from the net (often free) • I don’t know how to use or interpret them • Neither do the pros! • Practice, practice, practice …

  4. Time Series Analysis • A time series is a set of data pairs • t is the time, x is the data value • Usually, times are assumed error-free • Data = Signal + Error • x can be anthing, e.g. brightness of variables star, time of eclipse, eggs/day from a laying hen

  5. Actual Mean = = expected value Standard deviation = expected rms difference from mean Estimated Average = estimated Sample standard deviation = estimated Basic properties of data x

  6. Average and sample standard deviation • Average • Sample standard deviation

  7. Method #1: world’s best • Eye + Brain: Look at the data! • Plot x as a function of t: Explore! • Scientific name: Visual Inspection • World’s best – but not infallible • Programs: • TS http://www.aavso.org • MAGPLOT http://www.aavso.org

  8. Method #2: Fourier Analysis • Period analysis and curve-fitting • Powerful, well-understood, popular • Programs • TS http://www.aavso.org • PerAnSo http://www.peranso.com

  9. Method #3: Wavelet Analysis • Time-frequency analysis • Old versions bad, new version good • Programs: • WWZ http://www.aavso.org • WinWWZ http://www.aavso.org

  10. Visual Inspection

  11. Let’s take a look

  12. Fourier Analysis

  13. Fourier analysis for period search • Match the data to sine/cosine waves • = frequency • Period = • Amplitude = A = size of fluctuation • Obvious choice is period; mathematically sound choice is frequency

  14. Null Hypothesis (important!) • Null hypothesis: no time variation at all • So = constant • So, • Quite important! Often neglected. Even the pros often forget this.

  15. Is it real? • Fit produces a test statistic under the null hypothesis • Is usually “ /degree of freedom” (d) • Linear: is significant (not just by accident) at 95% confidence • 95% confidence means 5% false-alarm probability

  16. Meaning of significance • Significance does not mean the signal is linear, sinusoidal, periodic, etc. • It only means the null hypothesis is incorrect, i.e., the signal is not constant • Important!!!

  17. Pre-whitening • If you find a significant fit, then subtract the estimated signal, leaving residuals • Analyze the residuals for more structure • This process is called pre-whitening

  18. How to choose frequency? • Test all reasonable values, get a “strength of fit” for each. Common is “chi-square per degree of freedom” (but there are many) • Plot frequency .vs. fit – the Fourier transform (aka periodogram, aka power spectrum)

  19. Fourier decomposition Any periodic function of period P (frequency ) can be expressed as a Fourier series:

  20. Fundamental + harmonics For a pure sinusoid, expect response at frequency For a general periodic signal at a given frequency, expect a fundamental component at , as well as harmonics at frequencies etc.

  21. Lots of Fourier methods • FFT: fast Fourier transform • Not just fast: it’s wicked fast • Requires even time spacing • Requires N=integer power of 2 • Beware! • DFT: discrete Fourier transform • Applies to any time sampling, but incorrect results for highly uneven (as in astronomy!) • Beware!

  22. Problems from uneventime sampling • Aliasing: false peaks, often from a periodic data density • Aliases at • Common in astronomy: data density have a period P = 1 yr = 365.2422 d, so • Solution: pre-whitening

  23. Aliasing

  24. Aliasing: UZ Hya

  25. Problems from uneventime sampling • Mis-calculation of frequency (slightly) and amplitude (greatly); sabotages prewhitening

  26. Solution: better Fourier methods (for astronomy) • Lomb-Scargle modified periodogram • Improvement over FFT, DFT • CLEAN spectrum • Bigger improvement • DCDFT: date-compensated discrete Fourier transform (this is the one you want) • CLEANEST spectrum: DCDFT-like for multiple frequencies

  27. DCDFT • Much better estimates of period, amplitude

  28. Let’s take a look • Peranso (uses DCDFT and CLEANEST) • Available from CBA Belgium • http://www.peranso.com

  29. Fourier transform (CLEANEST) of TU Cas

  30. Wavelet Analysis

  31. Wavelets • Fit sine/cosine-like functions of brief duration • Shift them through time • Gives a time-frequency analysis

  32. Problems • Same old same old: uneven time spacing, especially variable data density, invalidate the results • But: even worse than Fourier • Essentially useless for most astronomical data

  33. Wavelet methods • DWT: discrete wavelet transform • Just not right for unevenly sampled data (astronomy!) • Solution: WWZ = weighted wavelet Z-transform

  34. Let’s take a look

  35. Data Analysis • Do it yourself • Use your eyes and brain • Healthy skepticism • tamino_9@hotmail.com • Enjoy!

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