<s>
Most	O
real	O
databases	B-Application
contain	O
data	O
whose	O
correctness	O
is	O
uncertain	O
.	O
</s>
<s>
This	O
is	O
achieved	O
by	O
using	O
probabilistic	B-General_Concept
databases	I-General_Concept
.	O
</s>
<s>
A	O
probabilistic	B-General_Concept
database	I-General_Concept
is	O
an	O
uncertain	B-Language
database	I-Language
in	O
which	O
the	O
possible	O
worlds	O
have	O
associated	O
probabilities	O
.	O
</s>
<s>
Probabilistic	B-General_Concept
database	I-General_Concept
management	O
systems	O
are	O
currently	O
an	O
active	O
area	O
of	O
research	O
.	O
</s>
<s>
"	O
While	O
there	O
are	O
currently	O
no	O
commercial	O
probabilistic	B-General_Concept
database	I-General_Concept
systems	O
,	O
several	O
research	O
prototypes	O
exist	O
...	O
"	O
</s>
<s>
Probabilistic	B-General_Concept
databases	I-General_Concept
distinguish	O
between	O
the	O
logical	B-Application
data	I-Application
model	I-Application
and	O
the	O
physical	O
representation	O
of	O
the	O
data	O
much	O
like	O
relational	B-Application
databases	I-Application
do	O
in	O
the	O
ANSI-SPARC	B-General_Concept
Architecture	I-General_Concept
.	O
</s>
<s>
In	O
probabilistic	B-General_Concept
databases	I-General_Concept
this	O
is	O
even	O
more	O
crucial	O
since	O
such	O
databases	B-Application
have	O
to	O
represent	O
very	O
large	O
numbers	O
of	O
possible	O
worlds	O
,	O
often	O
exponential	O
in	O
the	O
size	O
of	O
one	O
world	O
(	O
a	O
classical	O
database	O
)	O
,	O
succinctly	B-Data_Structure
.	O
</s>
<s>
In	O
a	O
probabilistic	B-General_Concept
database	I-General_Concept
,	O
each	O
tuple	O
is	O
associated	O
with	O
a	O
probability	O
between	O
0	O
and	O
1	O
,	O
with	O
0	O
representing	O
that	O
the	O
data	O
is	O
certainly	O
incorrect	O
,	O
and	O
1	O
representing	O
that	O
it	O
is	O
certainly	O
correct	O
.	O
</s>
<s>
A	O
probabilistic	B-General_Concept
database	I-General_Concept
could	O
exist	O
in	O
multiple	O
states	O
.	O
</s>
<s>
There	O
are	O
essentially	O
two	O
kinds	O
of	O
uncertainties	O
that	O
could	O
exist	O
in	O
a	O
probabilistic	B-General_Concept
database	I-General_Concept
,	O
as	O
described	O
in	O
the	O
table	O
below	O
:	O
</s>
<s>
The	O
first	O
published	O
use	O
of	O
the	O
term	O
"	O
probabilistic	B-General_Concept
database	I-General_Concept
"	O
was	O
probably	O
in	O
the	O
1987	O
VLDB	O
conference	O
paper	O
"	O
The	O
theory	O
of	O
probabilistic	B-General_Concept
databases	I-General_Concept
"	O
,	O
by	O
Cavallo	O
and	O
Pittarelli	O
.	O
</s>
<s>
The	O
title	O
(	O
of	O
the	O
11	O
page	O
paper	O
)	O
was	O
intended	O
as	O
a	O
bit	O
of	O
a	O
joke	O
,	O
since	O
David	O
Maier	O
's	O
600	O
page	O
monograph	O
,	O
The	O
Theory	O
of	O
Relational	B-Application
Databases	I-Application
,	O
would	O
have	O
been	O
familiar	O
at	O
that	O
time	O
to	O
most	O
of	O
the	O
conference	O
participants	O
and	O
readers	O
of	O
the	O
conference	O
proceedings	O
.	O
</s>
