Each location has to pick one of the Z decay exercises:
DELPHI ( Hands-on-CERN   or   Keyhole ) or
OPAL (Identifying particles)

It would be very desirable, if we had each day
both, DELPHI and OPAL events covered:
=> since OPAL("Identifying particles") is available in less languages
   (english, german, greek, italian, slovak)
   institutes from UK,FI,DE,GR,IT and SK should preferentially
   chose the OPAL package

Each exercise package contains 1000 events which should be 
distributed amongst the students,e.g.:
For 10 groups a 2 students: each group analyses 100 events
(see Zdecay_xxxx100.xls, xxx=DELPHI/OPAL local combination files)
For 20 groups a 2 students: each group analyses 50 events
(see Zdecay_xxxx50.xls, xxx=DELPHI/OPAL local combination files)
any other distribution is also possible, however, data sizes
smaller than 50 events/ group are not recommended (too few leptons).

If identical data samples are given to different groups, their 
results should be averaged, before entering the local EXCEL table.
NB: the EXCEL file does *not* require that the groups work through all
of their events. Often some careful groups are much more slower
than the others. Then either the faster groups have to get another
voluntary exercise or the slower groups just give e.g. the result
for 70 scanned events instead of 100.  

Please note that OPAL data have a cos theta cut in order
to suppress Bhabha e+e- scattering w.r.t. Z0 decays to e+e-.
An extra correction factor of 1.6 is applied in Zdecay_OPAL.xls 
to account for this cut. No such correction has to be applied to
DELPHI data, which cover a smaller angular range (Zdecay_DELPHI.xls)
and are thus less exposed to Bhabha background.

The decay probabilities ("branching ratios") are calculated in the
excel files w.r.t the *visible* decays. They are not corrected
for the 20% invisible Neutrino decays of the Z. For the discussion,
(see text file on discussion) the neutrino decays are irrelevant.

The presence of graduate student tutors for helping the high school
students with their PC exercises is extremely important. For about each
10 students (5 groups) one graduate student as tutor is needed.

In the 1/2h introduction to the exercise (which can be done e.g.
by browsing together through the first pages of teh exercise package
using a beamer) the scientist should address all important issues, 
but can for sure not cover all eventualities (e.g. muons appearing
in jets, converted photons from pi0 on tau decays, etc.). The students
should encounter problems themselves. Especially the "tau-tau" class
should not just used as "all what doesnt fit elsewhere", but
the students should be convinced about their choice.