What is "eyeballing the data"?
Drawing conclusions from data by simply looking at the data, either in raw data form, or as expressed in graphs or charts.
Example: Incomplete Regression
Graphical representations of data along with descriptive statistics should most always be a part of the initial review and analysis of data. But most of the time, conclusions should not be based solely on a visual review of the data, but should also be supported by a test of hypothesis or other equivalent method.
The eyeballing game: http://woodgears.ca/eyeball
Here are some notes, albeit tinged with a bit of sarcasm, on the perils of simply looking at data or simple graphics to draw conclusions. The link below is to the R Help mailing list, and a thread that discusses the IOTT (inter-ocular trauma test).
Other terms that might describe the same issue:
Candling the data (analogous to candling eggs)
Visualizing financial data
Vincent Granville
Many time series charts seem to exhibit a pattern: an up-trend, apparent periodicity, a stochastic process that seems not to be memory-less, and so on. Look at the following picture, representing stock price simulations. Do you think there is an up-trend?
Detecting Patterns with the Naked Eye
Follow-on comment by Alex Tolley
To add to that image, I have added a triangle chart pattern: http://www.flickr.com/photos/38563808@N00/11315285494/
A few years ago I wrote a program to add a number of common chart patterns to price series. What amazed me was how many patterns could be overlaid on a series, all conforming to strict rules of construction. Not only do our eyes play tricks, but we can easily write software to enhance those pattern finding illusions.