How The
Optimal Classification Scaling (OC) Program Works In One Dimension:

The "Edith" Algorithm

7 May 2001

I first started working on modeling roll call voting in 1978 when I was at the University of Oregon. I developed a simple scaling program that I dubbed

YYYYYYYYYYYYYYYYYNNNNNNNNNNNNNNNNNN * NNNYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY * YYYYYYYYNNNNNNNNNNNNNNNNNNNNNNNNNNN * Etc.

The asterisks indicate the "cutting point" for the roll call; that is, the midpoint of the Yea and Nay outcomes. A legislator located exactly on the midpoint would be indifferent between voting Yea or voting Nay.

Suppose roll call voting is in accord with this model. Then the scaling problem
consists of taking a roll call matrix and "unscrambling" it – that is, finding
a rank ordering of legislators and the correct **"polarity"**
(**Yea** to the left of the cutpoint or
**Yea** to the right of the cutpoint) for each roll call such
that patterns like those above are produced for each roll call. This is what
**Edith**
is designed to do. If the data is perfect, then the solution is easy and the correct
rank ordering is always found (see
Proof that if Voting is Perfect in One Dimension, then the First Eigenvector
Extracted from the Double-Centered Transformed Agreement Score Matrix has the
Same Rank Ordering as the True Data).

However, when there is error in the data, things get a bit complicated. When
there is error the aim is to find a rank ordering that maximizes the correct
classification of the observed Yeas and Nays. This is easier said than done because
if there are n legislators, then there are n!/2 possible rank orders to check to find
the best one. For example, for 50 legislators this number is about
1.52 * 10^{64} – a formidable number even with modern supercomputers.
Consequently,
**Edith**
embodies a "sensible" search procedure (what the Operations Researchers
call a "Heuristic") to find a solution. Namely, a good starting rank order
of the legislators is generated using the technique discussed for the perfect voting
problem (
Proof that if Voting is Perfect in One Dimension...)
and the corresponding cutting points are found.
These cutting points are used to get a new rank ordering of the legislators, and so on.
At each step the correct classification increases until a rank order is found that
produces cutting points that in return reproduce the rank order.

Finding the optimal rank ordering requires three steps:

- First, generate a starting estimate of the rank order of the legislators using
the method cited above and translate this rank ordering into evenly spaced coordinates
between -1.0 and +1.0;
- Second, given these coordinates, find the best cutting point for each roll call
that maximizes correct classification;
- Third, given these cutting points, find a new coordinate for each legislator
that maximizes correct classification.

To see how this process works, let the current ordering of the legislators
from left to right be represented by coordinates from **x _{1}** to

**
-1 £
x _{1 }£
x_{2 }£
x_{3 }£
... £
x_{p }£
+1.**

**
(-1, x _{1}), (x_{1} , x_{2}), ... , (x_{p} , +1)**

Actual Voting Pattern

Y Y Y Y Y Y . . . . Y Y * N Y * N Y * N N . . .N N

___________________________________________________________

-1.0 x_{1} x_{2} x_{3} x_{4} . . . . . . . 0.0 . . . . . . . x_{p-1} _{ }x_{p} +1.0

**Perfect Voting Patterns
(-1 , x**

Since there are only 2p perfect patterns, it is a simple matter to compare each
perfect pattern with the actual pattern of votes. This can be done very efficiently
by first assuming that the cutting point is in the region **(-1 , x _{1})** and
calculating the corresponding number of correct classifications. Next assume that
the cutting point is in the region

Let the ordering of the q roll call midpoints estimated above be represented by
coordinates from **z _{1}** to

**
-1 £
z _{1 }£
z_{2 }£
z_{3 }£
... £
z_{q }£
+1.**

**LLLLLLLLLCCCCCCCCCCCCCCCCCCCCCCCCC
or
LLLLLLLLLLLLLLLLLLLLLLLLLLLLLLCCCCCC**

The Figure below shows that by taking the polarity of the roll calls into account the legislator problem is equivalent to the roll call cutting point problem. Hence, the algorithm described above can be used to find the optimal legislator location. There are only 2q possible perfect patterns and only 2q calculations are required to find the maximum classification location for each legislator.

Actual Voting Pattern

L L L L L L . . . . L L * C L * C L * C C . . .C C

___________________________________________________________

-1.0 z_{1} z_{2} z_{3} z_{4} . . . . . . . 0.0 . . . . . . . z_{q-1} _{ }z_{q} +1.0

**Perfect Voting Patterns
(-1 , z**

This simple algorithm converges very rapidly to a solution in which the rank
ordering of the legislators and the rank ordering of the roll call midpoints reproduce
each other. This is a very strong form of ** conditional global maximum**.
Technically, if there are multiple sets of parameters (for example, as in this case,
parameters corresponding to rows of a matrix and parameters corresponding to columns
of a matrix), and every set of parameters is at a global maximum conditioned on the
other sets being held fixed, and these sets reproduce each other, then they are at a
conditional global maximum. Note that the overall global maximum, by definition,
is a conditional global maximum.

Conditional Global maxima are quite rare because of the nature of the constraints. Indeed, in the metric similarities problem, conditional global minima are very rare and their number declines as the number of parameters increases (Poole, 1990).

A simple illustration of how- Suppose we start with the scrambled ordering of the legislators shown below. Using the
method described above, we would place the Ace at the left end of the dimension because
this results in only one classification error. Placing it between the Ace and Two of
diamonds would produce four classifcation errors -- the Six, Seven, Four, and Nine of
Diamonds. The same logic holds for the Two and Three of Spades. No interior point does
better. When we get to the Four of Spades, however, placing it between the Three and Eight
of Diamonds produces four classification errors (the Six, Seven, Nine and Five of Diamonds).
The Four of Spades could be placed at the left end also producing four errors. In the
case of a tie, I always put the cutpoint/legislator at the interior most position.

- With respect to the ordering of the cutpoints represented by the Ace to the Nine of Spades,
the new legislator ordering is shown below. The Ace of Diamonds is placed to the left of
the Ace, Two, and Three of Spades, the Two and Three of Diamonds are placed at the same
position as the Ace, Two, and Three of Spades because of the tied ranks. The Four of
Diamonds is placed between the Ace, Two, Three stack, and the Four to Seven stack. The
Five, Six, Seven of Diamonds are placed at the same position as the Four to Seven stack of
Spades. The Eight of Diamonds is placed between the Four to Seven stack and the Eight and
Nine of Spades. The Nine of Diamonds is placed at the same position as the Eight and
Nine of Spades. Finally, the Ten of Diamonds is place to the right of the Eight and Nine of
Spades.

- With respect to the ordering of the legislators represented by the Ace to Ten of Diamonds,
the new ordering of the cutpoints is shown below. The Ace of Spades is placed between
the Ace of Diamonds and the Two and Three of Diamonds. The Two of Spades is placed at
the same position and the tied Two and Three of Diamonds. The Three of Spaces is placed
between the tied Two and Three of Diamonds and the Four of Diamonds. The Four of Spaces
is placed between the Four of Diamonds the the tied Five to Seven stack. The Five and
Six of Spaces are placed at the same position as the Five to Seven stack of Diamonds.
The Seven of Spades is placed between the Five to Seven stack and the Eight of Diamonds.
The Eight of Spades is placed between Eight and Nine of Diamonds. Finally, the Nine of
Spades is placed between the Nine and Ten of Diamonds.

- With respect to the ordering of the roll cutpoints represented by the Ace to Nine of Spades,
the ordering of the legislators represented by the Ace to Ten of Diamonds is shown below.
All are now in the correct position with perfect classification.

NOMINATE Data, Roll Call Data, and Software

Course Web Pages: University of Georgia (2010 - )

Course Web Pages: UC San Diego (2004 - 2010)

University of San Diego Law School (2005)

Course Web Pages: University of Houston (2000 - 2005)

Course Web Pages: Carnegie-Mellon University (1997 - 2000)

Recent Working Papers

Analyses of Recent Politics

About This Website

K7MOA Log Books: 1960 - 2017

Bio of Keith T. Poole

Related Links