I see many observers do not take the advice on page 38 in the AAVSO Manual for Visual Observing of Variable Stars (or here http://www.aavso.org/suggested-observing-cadences-variable-star-types) about the cadence for observing semi-regulars. Some observers have been observing Z UMa on many successive days, rather than once every 5 to 10 days.
Does this not bias the light-curve?
These are suggested cadences. It depends on the science that one is doing.
While these guidelines show 1 per day for AGN, several years ago during an outburst, I observed an AGN for about 3 hours at a cadence of once a minute, at the request of the collaborating astronomer. It clouded out at my observatory. An observer at another part of the world had complementary clear skies. Between us, we had the brightening and fading of the outburst. Neither of us had enough information to publish. However, with both of our data, it was sufficient to immediately publish the time scale of the outburst. According to the professional astronomer collaborator, it was an important result.
So it depends upon what science you are doing.
You might be right in some extent for AGNs, but observing an SR variable visually on many successive days realy does bias the light curve...
5 days cadence for Z UMa is fair enough. Meanwhile other binocular variables could be observed too...
Z UMa changes about a magnitude in 20 days, so observations every 4-5 days would make sense for visual observers. Of course, this biases the observations in the opposite sense: you expect to see your new estimate to be different from the previous one and question your measurement if they are the same.
However, I think Robert's comment is even more important: "Meanwhile other binocular variables could be observed." Z UMa is a well-observed variable, so for a single observer to monitor it every night doesn't fill in the light curve appreciably. At the same time, this means 4 or 5 estimates that could have been made on other stars, where additional measures are important. We have few enough visual observers as it is; when possible, they should optimize their observing to provide the most coverage.
That said, we really don't know the circumstances of the observer in question. They may be observing from their apartment/flat balcony and only have a limited sky area for observation. Or, they may be a beginner who can recognize the Z UMa field but have not yet graduated to other stars. Or, they may have a scientific project to see how well their estimates match the mean curve and so want to get dense coverage. So there may be reasons why someone is over-observing a star, but in most cases, it is likely that they just aren't thinking. If you know the observer in question, you might contact him/her and start a dialog.
The OP's question is one that I have brought up several times previously because of the potential harm a single observer's excessive observation of a particular variable can result in. Nor is this situation anything new. I've seen this errant practice done by a number of observers over the years, several of whom certainly knew better. Since the proper procedure is clearly outline in our literature, I can only attribute this practice to mostly an attempt to enhance one's observing totals.
I will admit that for heavily observed stars like Z UMa a single observer's excessive observations can only skew the lightcurve to a very slight degree normally. But follow the same practice in observing an otherwise much more infrequently covered star and the lightcurve can be thrown out of wack to a very significant degree, especially should the observer in question happen to display a major bias in his/her estimates (and I've witnessed a number that have!).
I know back in the time of Margaret Mayall she would either contract the offending observer and advise them to do it the right way, or simply drop out any excessive data, retaining only one estimate per appropriate interval from the observer in question. As our visual observer corp shrinks, this situation will increasingly become a problem unless addressed in some manner.
While I can relate to the issue at hand, for me it does not play a significant role in my observations. I generally observe eruptive variables or other irregular types, so there is no expectation of a magnitude to be had. Sometimes I observe those LPV's which have very few observations, but I do them fairly infrequently, so I lose track of where they are expected to be in my memory, and so again I do not have to deal with this problem.
The problem is then most acute for those observers who concentrate on a small number of LPV's and thus become very familiar with the expected behavior. And in this case, bias could be a significant factor. The easiest solution for those observers is to either (1) observe those few LPV's less frequently or (2) observe a larger number of different ones.
Or, if none of these suggestions works, then just wait until Alzheimer's sets in ;)
There really are no valid excuses for obervers who report estimates of the same SR variable every night, especially such a well-observed star as Z UMa. Even if the observer is new or suffers from limited sky access, he/she can still follow AAVSO recommended procedure and make an estimate once per week. There is no rule that an observer has to report observations every clear night. Just get a grip and limit your frequency of estimates. There are so many variables, so few observers, and so little time; curb your enthusiasm and spread your efforts.
YES, I have found in my section of the world that some folks forget the past and how things are suppose to be and how to conduct yourself in a professional manner. I have been chastised for prodding some folks in our organization. Just remember, A volunteer is a volunteer and should not be fired. One just hopes that people will learn from the past and perform properly.
I guess it's useless to talk about the science, biasing light curves and that sort of thing. How about informing observers who are estimating a popular SR variable every night that they are simply wasting their time? They could be observing a neglected star (charts are now free online); instead, they are repeating an observation they made last night, an observation that is neither needed nor useful.