I have carried out seismic response spectrum analysis , and have few reports copied to XLS . Further I have filtered these reports and created jpg files which I have uploaded.Names are X,Y,Z . Also I have uploaded one jpg file titled as included mass.
Queries are as under-

When filtered on X direction shock - it is observed in file "X" that for some frequencies the participation factor is negative. For first few frequency values , it is more than 1, and then it diminishes.What is the meaning of participation factor magnitude i.e. >1 or less than 1 as well as what is the meaning of it's sign , i.e. positive or negative?

In Y and Z direction shocks also this phenomenon is observed. For Z direction shock, the sign is not changing as it was changing frequently for X direction shocks.

In "included mass" report - % mass included is very less , whereas force added ( in my opinion , it is missing mass correction) percentage is very high. What is ideal percentage of "mass included"?

Richard /Dave , could you please guide by answering above query ?
Got an interview tomorrow !
Thx in advance.

These participation factors are used by the processor in calculating the individual modal response to your dynamic event. For the most part, it is the mass matrix pre- and post-multiplied by the mode shape.
There is no absolute maximum value nor do they add up to some set value. The bigger the (absolute) value, the more that mode is affected by the applied load vector.
Regarding the missing mass - it would be best to stop the modal contributions when the the remaining modes respond in a rigid fashion. Those higher modes would act together and that portion of the system mass responds in unison. That's why seismic analysis usually has a cutoff frequency of 33 Hz - it is assumed that the ground cannot move "fast enough" to cause any resonant response (i.e., DLF>1) in modes higher than 33 Hz. But response to other types of response spectrum analysis (e.g., DLF spectrum evaluation of a hammer event) may be difficult to predict since they are sensitive to load location and direction.