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Unit of Comparison Calculation

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GroundSwell

Sophomore Member
Joined
Apr 6, 2003
Professional Status
Certified General Appraiser
State
California
I remember from an AI courses that there's a simple calculation where you can analyze two sets of data and conclude what the best unit of comparison is (i.e. $/SF versus $/Unit). I've been racking my brain, but can't remember it for the life of me. Any help is appreciated.
 
Average, standard deviation, and the coefficient of variation?
 
The coefficient of variation sounds familiar. I'm pretty sure it was in Advanced Applications where they asked several of these types of questions. They'd give you two sets of data and you'd calculate a number for each set. If my memory serves me correct, whatever set had the lowest number was considered the best unit of comparison (mathematically at least).
 
I am not familiar with the course you are talking about but simply using common sense tells me the best unit of comparison is the one with the lowest percentage spread between the high and low ends of the unit ranges. I imagine you could build a formula but the easiest way for me to think about it would be to enter in the low unit number hit percentage change to the high (this of course assumes you use a HP-12C). Which ever has the lowest number is the best unit of comparison.

All of this assumes you have no market reference (for those who reply it is market driven). By the way welcome to the forum.
 
UNITS OF COMPARISON
Units of comparison are the components into which a property may be divided in order to make
comparisons. They are used to compare the subject and comparable properties. The appraiser
computes like units for comparison by stating each sale price in terms of appropriate units of
comparison. The appropriate unit depends on the type of property being appraised. For example,
apartments are typically compared based on sale price per apartment unit; commercial properties
based on sale price per square foot; industrial properties based on sale price per square or cubic
foot; hotels and motels based on sale price per guest room; etc. In the case of single-family
residences, the unit of comparison is typically the entire property. Units of comparison should be
selected based on what is typically used by buyers and sellers in the market for the type of
property being appraised. All applicable units of comparison should be considered, and any wide
variations in the results should be analyzed.
Sales data adjustments for elements of comparison can be made to either the total sale price or to
the appropriate unit(s) of comparison. Typically, adjustments for the rights and interests
conveyed, cash equivalence, and market conditions (and perhaps also non-real property items)
are made to the total sale price. Units of comparison are then calculated based on the adjusted
sale price. The unit of comparison is then adjusted for differences in location and physical and
economic characteristics.
 
More advance version

ELEMENTS OF COMPARISON
It is unlikely that the appraiser will find sales data so closely comparable to the subject property
that no adjustments will be required. The types of differences for which adjustments are often
required are referred to as elements of comparison. Elements of comparison, which are
enumerated in section 402.5 (above) and also in Rule 4, are the important factors that should be
separately considered and adjusted for, if necessary, when analyzing comparable properties.36
Rule 4 provides that when using the sales prices of the appraisal subject or of comparable
properties in valuation, the assessor shall:
(a) Convert a non-cash sale price to its cash equivalent by estimating the value in
cash of any tangible or intangible property other than cash which the seller
accepted in full or partial payment for the subject property and adding it to the
cash portion of the sale price and by deducting from the nominal sale price any
amount which the seller paid in lieu of interest to a lender who supplied the
grantee with part or all of the purchase money.
(b) When appraising an unencumbered fee interest, (1) convert the sale price of a
property encumbered with a debt to which the property remained subject to its
unencumbered fee price equivalent by adding to the sale price of the seller’s
equity the price for which it is estimated that such debt could have been sold
under value indicative conditions at the time the sale price was negotiated and (2)
convert the sale price of a property encumbered with a lease to which the property
remained subject to its unencumbered fee price equivalent by deducting from the
sale price of the seller’s equity the amount by which it is estimated that the lease
enhanced that price or adding to the price of the seller’s equity the amount by
which it is estimated that the lease depressed that price.
(c) Convert a sale to the valuation date of the subject property by adjusting it for
any change in price level of this type of property that has occurred between the
time the sale price was negotiated and the valuation date of the subject property.
(d) Make such allowances as he deems appropriate for differences between a
comparable property at the time of sale and the subject property on the valuation
date, in physical attributes of the properties, location of the properties, legally
enforceable restrictions on the properties’ use, and the income and amenities
which the properties are expected to produce. When the appraisal subject is land
and the comparable property is land of smaller dimensions, and it is assumed that
the subject property would be divided into comparable smaller parcels by a
purchaser, the assessor shall allow for the cost of subdivision, for the area
required for streets and alleys, for selling expenses, for normal profit, and for
interest charges during the period over which it is anticipated that the smaller
properties will be marketed.

Elements of comparison that must be considered by the appraiser are summarized as follows:
1. Property rights and interests conveyed;
2. Cash equivalence;
3. Non-real property items included in the sale, such as tangible personal property (e.g.,
equipment and furnishings) and non-taxable intangible assets and rights;
4. Market conditions;
5. Highest and best use and legally enforceable restrictions; and,
6. Location and physical and economic characteristics.
 
The coefficient of variation sounds familiar. I'm pretty sure it was in Advanced Applications where they asked several of these types of questions. They'd give you two sets of data and you'd calculate a number for each set. If my memory serves me correct, whatever set had the lowest number was considered the best unit of comparison (mathematically at least).

Simple calculation on the 12C to determine the average of the sample and its standard deviation. The standard deviation divided by the average gives you the coefficient of variation. It (the c of v) will tell you how consistent/dispersed the data is for the unit of comparison. The lower the coefficient of variation the more consistent the sample is (mathematically speaking).
 
Simple calculation on the 12C to determine the average of the sample and its standard deviation. The standard deviation divided by the average gives you the coefficient of variation. It (the c of v) will tell you how consistent/dispersed the data is for the unit of comparison. The lower the coefficient of variation the more consistent the sample is (mathematically speaking).

While indicative I think percentage differences, at least in practice, would be faster and for all intents yield the same selection. Maybe if there were large sample sizes an error would result but when you are only using 4 to 8 comps the faster method would prove adequate.

For full disclosure I understand you merely showing the process and not advocating a method. Simply bored. I should have gone on vacation slow as hell this week. I have six inspections and have not gotten one call back this week. Everyone is out of town
 
While indicative I think percentage differences, at least in practice, would be faster and for all intents yield the same selection. Maybe if there were large sample sizes an error would result but when you are only using 4 to 8 comps the faster method would prove adequate.

For full disclosure I understand you merely showing the process and not advocating a method. Simply bored. I should have gone on vacation slow as hell this week. I have six inspections and have not gotten one call back this week. Everyone is out of town

Actually the method I proposed gives one a better sense of where the indicators fall relative to a central tendency--think it is superior to the range of all inputs.
 
Actually the method I proposed gives one a better sense of where the indicators fall relative to a central tendency--think it is superior to the range of all inputs.

Here is what I will give you. It is more accurate. However, it is slower and would seldom yield different results from the spread.
 
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