Appraisers call this Absorption Rate! The numbers below would be similiar for the 1004MC form appraiser use.  (Learn more). 

Another method is simple inventory, however it assumes all the active inventory will sell, which it does not. Therefore this method will slightly overstate the inventory, but is more accurate than using the DOM, or Cumulative DOM as reported in MLS.  This method also counts the days from "Off Market" to "Close Date", however it will ALWAYS do this, therefore the trend is the same. 

Note:  Ada & Canyon is running about 40 days from "Off Market" to "Close Date"

MLS will provide the DOM of typically the LAST listing, not the 3 or 4 prior listings. This is more apparent in a downward market.  I would like to share detail examples, but it violates IMLS rules and regulations. (per IMLS)  However, agents which have access to IMLS can search the following addresses to understand the issue more.   

      1) 2301 W Hillway
      2) 7490 N Waterlilly Ave. 
      3) 25933 Chandler Drive 
      4) 3272 S Quarterswing Way
      5) 2030 W Havenwood Ave
      6) 1862 Grand Teton
      7) 12458 N Upper Ridge
      8) 304 S Rivermont Ln

Another Simple Example.....As of 01/01/2012

Six Months Ada County Sales (3,205 sales)
       Avg DOM = 76
       Median DOM = 41

Current Active Inventory in Ada County (2,403 listings)
     Avg DOM = 106
     Median DOM = 75

Therefore, ACTIVE inventory is 39% higher than SOLD inventory. (106/76=139%)   This shows, do not trust the DOM on sold inventory, it understates the "real" value.

Inventory is one key in understanding if a market is adding, reducing or holding steady. Inventory impacts days on the market (DOM), inventory along with the average sold price and average sales ratio are tools to understand the market.

Where does Ada County stand? Depends on what market.  Ada County overall inventory as Jan 1, 2012 is about 4.9 months, just looking at single family homes, about 4.7 months.  Keep in mind, this included the time from "Off Market Date" to "Close Date", which averages 40 days or 1.3 months. 

On average, a single family home in Ada County is getting offers in  3.4 months, Canyon County is 3.1 months from list date to offer date. 

Since "To Be Built" will distort the inventory, below is the summary data  on 01/01/2012.
                               Active      Pending    Percent Pending
          Ada               182         28                 13%
         Canyon          77           6                    10%

	Data Source : Intermountain MLS
	Report Date: January 1, 2012
	Date Filter: 1/1/2011 - 12/31/2011
	www.hennesseyappraisals.com
	County	Status & Est Inventory	TYPE OF LISTING				TOTAL
			Single Family	Single Family with Acreage	Condo	Townhouse
	Ada	Sold	6237	241	234	320	7032
		Active	1807	135	112	89	2143
		Pending	647	22	14	23	706
		Inventory*	4.7	7.8	6.5	4.2	4.9
	Canyon	Sold	2902	367	0	73	3342
		Active	738	144	2	16	900
		Pending	321	63	0	3	387
		Inventory*	4.4	6.8	N/A	3.1	4.6
	*Estimated from 12 months sales. 12 Month Sales / (Current Active + Current Pending )= Inventory
	Inventory Units = Months
	Single Family Only
	County	MLS Area	Current Active/Pend Listings	12 Month Sales	Est Inventory using 12 Months Sales (Months)	6 Month Sales	Est Inventory using 6 Months Sales (Months)	3 Month Sales	Est Inventory using 3 Months Sales (Months)
	Ada	Several	2454	6237	4.7	3199	4.6	1447	5.1
	Canyon	Several	1059	2902	4.4	1374	4.6	625	5.1
	Gem	Several	55	151	4.4	73	4.5	36	4.6
	Elmore	Several	199	208	11.5	114	10.5	59	10.1
	Payette	Several	127	187	8.1	100	7.6	44	8.7
Single Family Only - Date Range: 1/1/2011- 12/31/2011 Intermountain MLS
County	City	MLS Area	Active / Pending	12 Month Sales	Est Inventory using 12 Months Sales (Months)	6 Month Sales	Est Inventory using 6 Months Sales (Months)	3 Month Sales	Est Inventory using 3 Months Sales (Months)
Ada	Boise	100	142	333	5.1	169	5.0	79	5.4
		200	83	190	5.2	104	4.8	48	5.2
		300	162	440	4.4	237	4.1	108	4.5
		400	156	449	4.2	232	4.0	96	4.9
		500	64	191	4.0	104	3.7	62	3.1
		550	249	573	5.2	295	5.1	133	5.6
		600	109	325	4.0	160	4.1	73	4.5
		650	205	654	3.8	341	3.6	149	4.1
		800	183	407	5.4	207	5.3	90	6.1
	Eagle	900	197	391	6.0	198	6.0	89	6.6
	Star	950	101	170	7.1	88	6.9	38	8.0
	Meridian	1000	110	242	5.5	127	5.2	51	6.5
		1010	24	57	5.1	27	5.3	11	6.5
		1020	196	524	4.5	292	4.0	147	4.0
		1030	304	892	4.1	436	4.2	187	4.9
	Kuna	1100	156	382	4.9	176	5.3	82	5.7
Canyon	Nampa	1250	170	495	4.1	236	4.3	110	4.6
		1260	278	756	4.4	368	4.5	166	5.0
		1270	196	529	4.4	234	5.0	100	5.9
	Melba	1265	3	10	3.6	4	4.5	3	3.0
	Caldwell	1275	84	219	4.6	107	4.7	53	4.8
		1280	212	605	4.2	280	4.5	126	5.0
	Middleton	1285	65	203	3.8	99	3.9	52	3.8
	Other	1290	13	27	5.8	16	4.9	6	6.5
Single Family Only - Date Range: 1/1/2011- 12/31/2011 Intermountain MLS
County	City	Price Range	Active	12 Month Sales	Est Inventory using 12 Months Sales (Months)	6 Month Sales	Est Inventory using 6 Months Sales (Months)	3 Month Sales	Est Inventory using 3 Months Sales (Months)
Ada	Several	$0 - $99,999	343	1315	3.1	620	3.3	278	3.7
		$100,00 - $199,999	1198	3241	4.4	1680	4.3	768	4.7
		$200,000 - $299,999	508	1052	5.8	559	5.5	252	6.0
		$300,000 - $449,999	230	447	6.2	245	5.6	113	6.1
		$450,000 - over	114	124	11.0	64	10.7	25	13.7
Canyon	Several	$0 - $99,999	620	2096	3.5	978	3.8	450	4.1
		$100,00 - $199,999	355	694	6.1	344	6.2	153	7.0
		$200,000 - $299,999	43	66	7.8	32	8.1	18	7.2
		$300,000 - $449,999	10	8	15.0	5	12.0	3	10.0
		$450,000 - over	2	0	N/A	0	N/A	0	N/A

	Single Family with ACREAGE
	Must have More than 1 Acre.
	County	MLS Area	Current Active/Pend Listings	12 Month Sales	Est Inventory using 12 Months Sales (Months)	6 Month Sales	Est Inventory using 6 Months Sales (Months)	3 Month Sales	Est Inventory using 3 Months Sales (Months)
	Ada	Several	200	299	8.0	160	7.5	74	8.1
	Canyon	Several	226	413	6.6	211	6.4	104	6.5
	Gem	Several	52	98	6.4	52	6.0	25	6.2
	Elmore	Several	53	42	15.1	25	12.7	16	9.9
	Payette	Several	61	56	13.1	26	14.1	10	18.3
Single Family with ACREAGE - Date Range: 1/1/2011 - 12/31/2011. Intermountain MLS
County	City	Price Range	Current Active/Pend Listings	12 Month Sales	Est Inventory using 12 Months Sales (Months)	6 Month Sales	Est Inventory using 6 Months Sales (Months)	3 Month Sales	Est Inventory using 3 Months Sales (Months)
Ada	Several	$0 - $99,999	2	18	1.3	10	1.2	4	1.5
		$100,00 - $199,999	27	60	5.4	27	6.0	14	5.8
		$200,000 - $299,999	36	68	6.4	40	5.4	20	5.4
		$300,000 - $449,999	45	86	6.3	51	5.3	21	6.4
		$450,000 - over	81	67	14.5	33	14.7	16	15.2
Canyon	Several	$0 - $99,999	21	77	3.3	43	2.9	21	3.0
		$100,00 - $199,999	71	167	5.1	80	5.3	41	5.2
		$200,000 - $299,999	71	123	6.9	67	6.4	33	6.5
		$300,000 - $449,999	33	44	9.0	21	9.4	9	11.0
		$450,000 - over	14	3	56.0	1	84.0	1	336.0